Darwinism (1889) Part 32

Having now set forth and ill.u.s.trated at some length the most important of the applications of the development hypothesis in the explanation of the broader and more generally interesting phenomena presented by the organic world, we propose to discuss some of the more fundamental problems and difficulties which have recently been adduced by eminent naturalists. It is the more necessary to do this, because there is now a tendency to minimise the action of natural selection in the production of organic forms, and to set up in its place certain fundamental principles of variation or laws of growth, which it is urged are the real originators of the several lines of development, and of most of the variety of form and structure in the vegetable and animal kingdoms.

These views have, moreover, been seized upon by popular writers to throw doubt and discredit on the whole theory of evolution, and especially on Darwin's presentation of that theory, to the bewilderment of the general public, who are quite unable to decide how far the new views, even if well established, tend to subvert the Darwinian theory, or whether they are really more than subsidiary parts of it, and quite powerless without it to produce any effect whatever.

The writers whose special views we now propose to consider are: (1) Mr.

Herbert Spencer, on modification of structures arising from modification of functions, as set forth in his _Factors of Organic Evolution_. (2) Dr. E.D. Cope, who advocates similar views in detail, in his work ent.i.tled _The Origin of the Fittest_, and may be considered the head of a school of American naturalists who minimise the agency of natural selection. (3) Dr. Karl Semper, who has especially studied the direct influence of the environment in the whole animal kingdom, and has set forth his views in a volume on _The Natural Conditions of Existence as they Affect Animal Life_. (4) Mr. Patrick Geddes, who urges that fundamental laws of growth, and the antagonism of vegetative and reproductive forces, account for much that has been imputed to natural selection.

We will now endeavour to ascertain what are the more important facts and arguments adduced by each of the above writers, and how far they offer a subst.i.tute for the action of natural selection; having done which, a brief account will be given of the views of Dr. Aug. Weismann, whose theory of heredity will, if established, strike at the very root of the arguments of the first three of the writers above referred to.

_Mr. Herbert Spencer's Factors of Organic Evolution._

Mr. Spencer, while fully recognising the importance and wide range of the principle of natural selection, thinks that sufficient weight has not been given to the effects of use and disuse as a factor in evolution, or to the direct action of the environment in determining or modifying organic structures. As examples of the former cla.s.s of actions, he adduces the decreased size of the jaws in the civilised races of mankind, the inheritance of nervous disease produced by overwork, the great and inherited development of the udders in cows and goats, and the shortened legs, jaws, and snout in improved races of pigs--the two latter examples being quoted from Mr. Darwin,--and other cases of like nature. As examples of the latter, Mr. Darwin is again quoted as admitting that there are many cases in which the action of similar conditions appears to have produced corresponding changes in different species; and we have a very elaborate discussion of the direct action of the medium in modifying the protoplasm of simple organisms, so as to bring about the difference between the outer surface and the inner part that characterises the cells or other units of which they are formed.

Now, although this essay did little more than bring together facts which had been already adduced by Mr. Darwin or by Mr. Spencer himself, and lay stress upon their importance, its publication in a popular review was immediately seized upon as "an avowed and definite declaration against some of the leading ideas on which the Mechanical Philosophy depends," and as being "fatal to the adequacy of the Mechanical Philosophy as any explanation of organic evolution,"[198]--an expression of opinion which would be repudiated by every Darwinian. For, even admitting the interpretation which Mr. Spencer puts on the facts he adduces, they are all included in the causes which Darwin himself recognised as having acted in bringing about the infinitude of forms in the organic world. In the concluding chapter of the _Origin of Species_ he says: "I have now recapitulated the facts and considerations which have thoroughly convinced me that species have been modified during a long course of descent. This has been effected chiefly through the natural selection of numerous successive, slight, favourable variations; aided in an important manner by the inherited effects of the use and disuse of parts; and in an unimportant manner--that is, in relation to adaptive structures whether past or present, by the direct action of external conditions, and by variations which seem to us, in our ignorance, to arise spontaneously." This pa.s.sage, summarising Darwin's whole inquiry, and explaining his final point of view, shows how very inaccurate may be the popular notion, as expressed by the Duke of Argyll, of any supposed additions to the causes of change of species as recognised by Darwin.

But, as we shall see presently, there is now much reason to believe that the supposed inheritance of acquired modifications--that is, of the effects of use and disuse, or of the direct influence of the environment--is not a fact; and if so, the very foundation is taken away from the whole cla.s.s of objections on which so much stress is now laid.

It therefore becomes important to inquire whether the facts adduced by Darwin, Spencer, and others, do really necessitate such inheritance, or whether any other interpretation of them is possible. I believe there is such an interpretation; and we will first consider the cases of disuse on which Mr. Spencer lays most stress.

The cases Mr. Spencer adduces as demonstrating the effects of disuse in diminishing the size and strength of organs are, the diminished size of the jaws in the races of civilised men, and the diminution of the muscles used in closing the jaws in the case of pet-dogs fed for generations on soft food. He argues that the minute reduction in any one generation could not possibly have been useful, and, therefore, not the subject of natural selection; and against the theory of correlation of the diminished jaw with increased brain in man, he urges that there are cases of large brain development, accompanied by jaws above the average size. Against the theory of economy of nutrition in the case of the pet-dogs, he places the abundant food of these animals which would render such economy needless.

But neither he nor Mr. Darwin has considered the effects of the withdrawal of the action of natural selection in keeping up the parts in question to their full dimensions, which, of itself, seems to me quite adequate to produce the results observed. Recurring to the evidence, adduced in Chapter III, of the constant variation occurring in all parts of the organism, while selection is constantly acting on these variations in eliminating all that fall below the best working standard, and preserving only those that are fully up to it; and, remembering further, that, of the whole number of the increase produced annually, only a small percentage of the best adapted can be preserved, we shall see that every useful organ will be kept up nearly to its higher limit of size and efficiency. Now Mr. Galton has proved experimentally that, when any part has thus been increased (or diminished) by selection, there is in the offspring a strong tendency to revert to a mean or average size, which tends to check further increase. And this mean appears to be, not the mean of the actual existing individuals but a lower mean, or that from which they had been recently raised by selection.[199] He calls this the law of "Regression towards Mediocrity," and it has been proved by experiments with vegetables and by observations on mankind. This regression, in every generation, takes place even when both parents have been selected for their high development of the organ in question; but when there is no such selection, and crosses are allowed among individuals of every grade of development, the deterioration will be very rapid; and after a time not only will the average size of the part be greatly reduced, but the instances of full development will become very rare. Thus what Weismann terms "panmixia," or free intercrossing, will co-operate with Galton's law of "regression towards mediocrity," and the result will be that, whenever selection ceases to act on any part or organ which has heretofore been kept up to a maximum of size and efficiency, the organ in question will rapidly decrease till it reaches a mean value considerably below the mean of the progeny that has usually been produced each year, and very greatly below the mean of that portion which has survived annually; and this will take place by the general law of heredity, and quite irrespective of any _use_ or _disuse_ of the part in question. Now, no observations have been adduced by Mr. Spencer or others, showing that the average amount of change supposed to be due to _disuse_ is greater than that due to the law of regression towards mediocrity; while even if it were somewhat greater, we can see many possible contributory causes to its production. In the case of civilised man's diminished jaw, there may well be some correlation between the jaw and the brain, seeing that increased mental activity would lead to the withdrawal of blood and of nervous energy from adjacent parts, and might thus lead to diminished growth of those parts in the individual. And in the case of pet-dogs, the selection of small or short-headed individuals would imply the unconscious selection of those with less ma.s.sive temporal muscles, and thus lead to the concomitant reduction of those muscles. The amount of reduction observed by Darwin in the wing-bones of domestic ducks and poultry, and in the hind legs of tame rabbits, is very small, and is certainly no greater than the above causes will well account for; while so many of the external characters of all our domestic animals have been subject to long-continued artificial selection, and we are so ignorant of the possible correlations of different parts, that the phenomena presented by them seem sufficiently explained without recurrence to the a.s.sumption that any changes in the individual, due to disuse, are inherited by the offspring.

_Supposed Effects of Disuse among Wild Animals._

It may be urged, however, that among wild animals we have many undoubted results of disuse much more p.r.o.nounced than those among domestic kinds, results which cannot be explained by the causes already adduced. Such are the reduced size of the wings of many birds on oceanic islands; the abortion of the eyes in many cave animals, and in some which live underground; and the loss of the hind limbs in whales and in some lizards. These cases differ greatly in the amount of the reduction of parts which has taken place, and may be due to different causes. It is remarkable that in some of the birds of oceanic islands the reduction is little if at all greater than in domestic birds, as in the water-hen of Tristan d'Acunha. Now if the reduction of wing were due to the hereditary effects of disuse, we should expect a very much greater effect in a bird inhabiting an oceanic island than in a domestic bird, where the disuse has been in action for an indefinitely shorter period.

In the case of many other birds, however--as some of the New Zealand rails and the extinct dodo of Mauritius--the wings have been reduced to a much more rudimentary condition, though it is still obvious that they were once organs of flight; and in these cases we certainly require some other causes than those which have reduced the wings of our domestic fowls. One such cause may have been of the same nature as that which has been so efficient in reducing the wings of the insects of oceanic islands--the destruction of those which, during the occasional use of their wings, were carried out to sea. This form of natural selection may well have acted in the case of birds whose powers of flight were already somewhat reduced, and to whom, there being no enemies to escape from, their use was only a source of danger. We may thus, perhaps, account for the fact that many of these birds retain small but useless wings with which they never fly; for, the wings having been reduced to this functionless condition, no power could reduce them further except correlation of growth or economy of nutrition, causes which only rarely come into play.

The complete loss of eyes in some cave animals may, perhaps, be explained in a somewhat similar way. Whenever, owing to the total darkness, they became useless, they might also become injurious, on account of their delicacy of organisation and liability to accidents and disease; in which case natural selection would begin to act to reduce, and finally abort them; and this explains why, in some cases, the rudimentary eye remains, although completely covered by a protective outer skin. Whales, like moas and ca.s.sowaries, carry us back to a remote past, of whose conditions we know too little for safe speculation. We are quite ignorant of the ancestral forms of either of these groups, and are therefore without the materials needful for determining the steps by which the change took place, or the causes which brought it about.[200]

On a review of the various examples that have been given by Mr. Darwin and others of organs that have been reduced or aborted, there seems too much diversity in the results for all to be due to so direct and uniform a cause as the individual effects of disuse acc.u.mulated by heredity. For if that were the only or chief efficient cause, and a cause capable of producing a decided effect during the comparatively short period of the existence of animals in a state of domestication, we should expect to find that, in wild species, all unused parts or organs had been reduced to the smallest rudiments, or had wholly disappeared. Instead of this we find various grades of reduction, indicating the probable result of several distinct causes, sometimes acting separately, sometimes in combination, such as those we have already pointed out.

And if we find no positive evidence of _disuse_, acting by its direct effect on the individual, being transmitted to the offspring, still less can we find such evidence in the case of the _use_ of organs. For here the very fact of _use_, in a wild state, implies _utility_, and utility is the constant subject for the action of natural selection; while among domestic animals those parts which are exceptionally used are so used in the service of man, and have thus become the subjects of artificial selection. Thus "the great and inherited development of the udders in cows and goats," quoted by Spencer from Darwin, really affords no proof of inheritance of the increase due to use, because, from the earliest period of the domestication of these animals, abundant milk-production has been highly esteemed, and has thus been the subject of selection; while there are no cases among wild animals that may not be better explained by variation and natural selection.

_Difficulty as to Co-adaptation of Parts by Variation and Selection._

Mr. Spencer again brings forward this difficulty, as he did in his _Principles of Biology_ twenty-five years ago, and urges that all the adjustments of bones, muscles, blood-vessels, and nerves which would be required during, for example, the development of the neck and fore-limbs of the giraffe, could not have been effected by "simultaneous fortunate spontaneous variations." But this difficulty is fully disposed of by the facts of simultaneous variation adduced in our third chapter, and has also been specially considered in Chapter VI, p. 127. The best answer to this objection may, perhaps, be found in the fact that the very thing said to be impossible by variation and natural selection has been again and again effected by variation and artificial selection. During the process of formation of such breeds as the greyhound or the bulldog, of the race-horse and carthorse, of the fantail pigeon or the otter-sheep, many co-ordinate adjustments have been produced; and no difficulty has occurred, whether the change has been effected by a single variation--as in the last case named--or by slow steps, as in all the others. It seems to be forgotten that most animals have such a surplus of vitality and strength for all the ordinary occasions of life that any slight superiority in one part can be at once utilised; while the moment any want of balance occurs, variations in the insufficiently developed parts will be selected to bring back the harmony of the whole organisation.

The fact that, in all domestic animals, variations do occur, rendering them swifter or stronger, larger or smaller, stouter or slenderer, and that such variations can be separately selected and acc.u.mulated for man's purposes, is sufficient to render it certain that similar or even greater changes may be effected by natural selection, which, as Darwin well remarks, "acts on every internal organ, on every shade of const.i.tututional difference, on the whole machinery of life." The difficulty as to co-adaptation of parts by variation and natural selection appears to me, therefore, to be a wholly imaginary difficulty which has no place whatever in the operations of nature.

_Direct Action of the Environment._

Mr. Spencer's last objection to the wide scope given by Darwinians to the agency of natural selection is, that organisms are acted upon by the environment, which produces in them definite changes, and that these changes in the individual are transmitted by inheritance, and thus become increased in successive generations. That such changes are produced in the individual there is ample evidence, but that they are inherited independently of any form of selection or of reversion is exceedingly doubtful, and Darwin nowhere expresses himself as satisfied with the evidence. The two very strongest cases he mentions are the twenty-nine species of American trees which all differed in a corresponding way from their nearest European allies; and the American maize which became changed after three generations in Europe. But in the case of the trees the differences alleged may be partly due to correlation with const.i.tutional peculiarities dependent on climate, especially as regards the deeper tint of the fading leaves and the smaller size of the buds and seeds in America than in Europe; while the less deeply toothed or serrated leaves in the American species are, in our present complete ignorance of the causes and uses of serration, quite as likely to be due to some form of adaptation as to any direct action of the climate. Again, we are not told how many of the allied species do not vary in this particular manner, and this is certainly an important factor in any conclusion we may form on the question.

In the case of the maize it appears that one of the more remarkable and highly selected American varieties was cultivated in Germany, and in three years nearly all resemblance to the original parent was lost; and in the sixth year it closely resembled a common European variety, but was of somewhat more vigorous growth. In this case no selection appears to have been practised, and the effects may have been due to that "reversion to mediocrity" which invariably occurs, and is more especially marked in the case of varieties which have been rapidly produced by artificial selection. It may be considered as a partial reversion to the wild or unimproved stock; and the same thing would probably have occurred, though perhaps less rapidly, in America itself.

As this is stated by Darwin to be the most remarkable case known to him "of the direct and prompt action of climate on a plant," we must conclude that such direct effects have not been proved to be acc.u.mulated by inheritance, independently of reversion or selection.

The remaining part of Mr. Spencer's essay is devoted to a consideration of the hypothetical action of the environment on the lower organisms which consist of simple cells or formless ma.s.ses of protoplasm; and he shows with great elaboration that the outer and inner parts of these are necessarily subject to different conditions; and that the outer actions of air or water lead to the formation of integuments, and sometimes to other definite modifications of the surface, whence arise permanent differences of structure. Although in these cases also it is very difficult to determine how much is due to direct modification by external agencies transmitted and acc.u.mulated by inheritance, and how much to spontaneous variations acc.u.mulated by natural selection, the probabilities in favour of the former mode of action are here greater, because there is no differentiation of nutritive and reproductive cells in these simple organisms; and it can be readily seen that any change produced in the latter will almost certainly affect the next generation.[201] We are thus carried back almost to the origin of life, and can only vaguely speculate on what took place under conditions of which we know so little.

_The American School of Evolutionists._

The tentative views of Mr. Spencer which we have just discussed, are carried much further, and attempts have been made to work them out in great detail, by many American naturalists, whose best representative is Dr. E.D. Cope of Philadelphia.[202] This school endeavours to explain all the chief modifications of form in the animal kingdom by fundamental laws of growth and the inherited effects of use and effort, returning, in fact, to the teachings of Lamarck as being at least equally important with those of Darwin.

The following extract will serve to show the high position claimed by this school as original discoverers, and as having made important additions to the theory of evolution:

"Wallace and Darwin have propounded as the cause of modification in descent their law of natural selection. This law has been epitomised by Spencer as the 'survival of the fittest.' This neat expression no doubt covers the case, but it leaves the origin of the fittest entirely untouched. Darwin a.s.sumes a 'tendency to variation' in nature, and it is plainly necessary to do this, in order that materials for the exercise of a selection should exist. Darwin and Wallace's law is then only restrictive, directive, conservative, or destructive of something already created. I propose, then, to seek for the originative laws by which these subjects are furnished; in other words, for the causes of the origin of the fittest."[203]

Mr. Cope lays great stress on the existence of a special developmental force termed "bathmism" or growth-force, which acts by means of r.e.t.a.r.dation and acceleration "without any reference to fitness at all;"

that "instead of being controlled by fitness it is the controller of fitness." He argues that "all the characteristics of generalised groups from genera up (excepting, perhaps, families) have been evolved under the law of acceleration and r.e.t.a.r.dation," combined with some intervention of natural selection; and that specific characters, or species, have been evolved by natural selection with some a.s.sistance from the higher law. He, therefore, makes species and genera two absolutely distinct things, the latter not developed out of the former; generic characters and specific characters are, in his opinion, fundamentally different, and have had different origins, and whole groups of species have been simultaneously modified, so as to belong to another genus; whence he thinks it "highly probable that the same specific form has existed through a succession of genera, and perhaps in different epochs of geologic time."

Useful characters, he concludes, have been produced by the special location of growth-force by use; useless ones have been produced by location of growth-force without the influence of use. Another element which determines the direction of growth-force, and which precedes use, is effort; and "it is thought that effort becomes incorporated into the metaphysical acquisitions of the parent, and is inherited with other metaphysical qualities by the young, which, during the period of growth, is much more susceptible to modifying influences, and is likely to exhibit structural change in consequence."[204]

From these few examples of their teachings, it is clear that these American evolutionists have departed very widely from the views of Mr.

Darwin, and in place of the well-established causes and admitted laws to which he appeals have introduced theoretical conceptions which have not yet been tested by experiments or facts, as well as metaphysical conceptions which are incapable of proof. And when they come to ill.u.s.trate these views by an appeal to palaeontology or morphology, we find that a far simpler and more complete explanation of the facts is afforded by the established principles of variation and natural selection. The confidence with which these new ideas are enunciated, and the repeated a.s.sertion that without them Darwinism is powerless to explain the origin of organic forms, renders it necessary to bestow a little more time on the explanations they give us of well-known phenomena with which, they a.s.sert, other theories are incompetent to grapple.

As examples of use producing structural change, Mr. Cope adduces the hooked and toothed beaks of the falcons and the butcher-birds, and he argues that the fact of these birds belonging to widely different groups proves that similarity of use has produced a similar structural result.

But no attempt is made to show any direct causal connection between the use of a bill to cut or tear flesh and the development of a tooth on the mandible. Such use might conceivably strengthen the bill or increase its size, but not cause a special tooth-like outgrowth which was not present in the ancestral thrush-like forms of the butcher-bird. On the other hand, it is clear that any variations of the bill tending towards a hook or tooth would give the possessor some advantage in seizing and tearing its prey, and would thus be preserved and increased by natural selection. Again, Mr. Cope urges the effects of a supposed "law of polar or centrifugal growth" to counteract a tendency to unsymmetrical growth, where one side of the body is used more than the other. But the undoubted hurtfulness of want of symmetry in many important actions or functions would rapidly eliminate any such tendency. When, however, it has become useful, as in the case of the single enlarged claw of many Crustacea, it has been preserved by natural selection.

_Origin of the Feet of the Ungulates._

Perhaps the most original and suggestive of Mr. Cope's applications of the theory of use and effort in modifying structure are, his chapters "On the Origin of the Foot-Structure of the Ungulates;" and that "On the Effect of Impacts and Strains on the Feet of Mammalia;" and they will serve also to show the comparative merits of this theory and that of natural selection in explaining a difficult case of modification, especially as it is an explanation claimed as new and original when first enunciated in 1881. Let us, then, see how he deals with the problem.

The remarkable progressive change of a four or five-toed ancestor into the one-toed horse, and the equally remarkable division of the whole group of ungulate animals into the odd-toed and even-toed divisions, Mr.

Cope attempts to explain by the effects of impact and use among animals which frequented hard or swampy ground respectively. On hard ground, it is urged, the long middle toe would be most used and subjected to the greatest strains, and would therefore acquire both strength and development. It would then be still more exclusively used, and the extra nourishment required by it would be drawn from the adjacent less-used toes, which would accordingly diminish in size, till, after a long series of changes, the records of which are so well preserved in the American tertiary rocks, the true one-toed horse was developed. In soft or swampy ground, on the other hand, the tendency would be to spread out the foot so that there were two toes on each side. The two middle toes would thus be most used and most subject to strains, and would, therefore, increase at the expense of the lateral toes. There would be, no doubt, an advantage in these two functional toes being of equal size, so as to prevent twisting of the foot while walking; and variations tending to bring this about would be advantageous, and would therefore be preserved. Thus, by a parallel series of changes in another direction, adapted to a distinct set of conditions, we should arrive at the symmetrical divided hoofs of our deer and cattle. The fact that sheep and goats are specially mountain and rock-loving animals may be explained by their being a later modification, since the divided hoof once formed is evidently well adapted to secure a firm footing on rugged and precipitous ground, although it could hardly have been first developed in such localities. Mr. Cope thus concludes: "Certain it is that the length of the bones in the feet of the ungulate orders has a direct relation to the dryness of the ground they inhabit, and the possibility of speed which their habit permits them or necessarily imposes on them."[205]

If there is any truth in the explanation here briefly summarised, it must entirely depend on the fact of individual modifications thus produced being hereditary, and we yet await the proof of this. In the meantime it is clear that the very same results could have been brought about by variation and natural selection. For the toes, like all other organs, vary in size and proportions, and in their degree of union or separation; and if in one group of animals it was beneficial to have the middle toe larger and longer, and in another set to have the two middle toes of the same size, nothing can be more certain than that these particular modifications would be continuously preserved, and the very results we see ultimately produced.

The oft-repeated objections that the cause of variations is unknown, that there must be something to determine variations in the right direction; that "natural selection includes no actively progressive principle, but must wait for the development of variation, and then, after securing the survival of the best, wait again for the best to project its own variations for selection," we have already sufficiently answered by showing that variation--in abundant or typical species--is always present in ample amount; that it exists in all parts and organs; that these vary, for the most part, independently, so that any required combination of variations can be secured; and finally, that all variation is necessarily either in excess or defect of the mean condition, and that, consequently, the right or favourable variations are so frequently present that the unerring power of natural selection never wants materials to work upon.

_Supposed Action of Animal Intelligence._

The following pa.s.sage briefly summarises Mr. Cope's position: "Intelligence is a conservative principle, and will always direct effort and use into lines which will be beneficial to its possessor. Here we have the source of the fittest, _i.e._ addition of parts by increase and location of growth-force, directed by the influence of various kinds of compulsion in the lower, and intelligent option among higher animals.

Thus intelligent choice, taking advantage of the successive evolution of physical conditions, may be regarded as the _originator of the fittest_, while natural selection is the tribunal to which all results of accelerated growth are submitted. This preserves or destroys them, and determines the new points of departure on which accelerated growth shall build."[206]

This notion of "intelligence"--the intelligence of the animal itself--determining its own variation, is so evidently a very partial theory, inapplicable to the whole vegetable kingdom, and almost so to all the lower forms of animals, amongst which, nevertheless, there is the very same adaptation and co-ordination of parts and functions as among the highest, that it is strange to see it put forward with such confidence as necessary for the completion of Darwin's theory. If "the various kinds of compulsion"--by which are apparently meant the laws of variation, growth, and reproduction, the struggle for existence, and the actions necessary to preserve life under the conditions of the animal's environment--are sufficient to have developed the varied forms of the lower animals and of plants, we can see no reason why the same "compulsion" should not have carried on the development of the higher animals also. The action of this "intelligent option" is altogether unproved; while the acknowledgment that natural selection is the tribunal which either preserves or destroys the variations submitted to it, seems quite inconsistent with the statement that intelligent choice is the "orginator of the fittest," since whatever is really "the fittest" can never be destroyed by natural selection, which is but another name for the survival of the fittest. If "the fittest" is always definitely produced by some other power, then natural selection is not wanted. If, on the other hand, both fit and unfit are produced, and natural selection decides between them, that is pure Darwinism, and Mr.

Cope's theories have added nothing to it.

[Ill.u.s.tration: FIG. 35.--Transformation of Artemia salina to A.

Milhausenii; 1, tail-lobe of A. salina, and its transition through 2,3,4,5, to 6, into that of A. Milhausenii; 7, post-abdomen of A.

salina; 8, post-abdomen of a form bred in brackish water; 9, gill of A.

Milhausenii; 10, gill of A. salina. (From Schmankewitsch.)]

_Semper on the Direct Influence of the Environment._