September 3, 1914] 



NATURE 



21 



of an adaptive character and will follow definite lines 

 There is good reason for believing, however, that 

 manv minor modifications in structure may arise and 

 persist, incidentally as it were, that have no signi- 

 ficance as adaptations. 



One of the most remarkable and distinctive features 

 f the lower vertebrates is the presence of gill-slits 

 ;? accessory organs of respiration. These gill-slits 

 are clearly an adaptation to aquatic life. When the 

 ancestors of the higher vertebrates left the water and 

 took to life on land the gills disappeared and were 

 replaced by lungs, adapted for air-breathing. The 

 chcuige must, of course, have been an extremely 

 gradual one, and we get a verj- clear indication of how 

 it took place in the surviving dipnoids, which have 

 remained in this respect in an intermediate condition 

 between the fishes and the amphibia, possessing and 

 using both gills and lungs. 



\\'e also know that even the most highly specialised 

 air-breathing vertebrates, which never live in water 

 and never require gills or gill-slits at all, nevertheless 

 possess ver\- distinct gill-slits during a certain period 

 of their development. This is one of the most familiar 

 illustrations of the law of recapitulation, and my only 

 excuse for bringing it forward now is that I wish, 

 before going further, to consider a difticulty — perhaps 

 more apparent than real— that arises in connection 

 with such cases. 



It might be argued that if gill-slits arose in response 

 to the stimuli of aquatic life, and if these stimuli 

 are no longer operative in the case of air-breathing 

 vertebrates, then gill-slits ought not to be develojjed 

 at any stage of their existence. This argument is, I 

 think, fully met by the following considerations. 



At any given moment of ontogenetic development 

 the condition of any organ is merely the last term 

 of a series of morphogenetic stages, while its environ- 

 ment at the same moment — which, of course, includes 

 its relation to all the other organs of the body — is 

 likewise merely the last term of a series of environ- 

 mental stages. We have thus two parallel series of 

 events to take into consideration in endeavouring to 

 account for the condition of any p)art of an organism 

 — or of the organism as a whole— at any period of its 

 existence : — 



Ej E^ E, ... ... ... E„ environmental stages 



Mj Mo M3 ... ... ... M„ morphogenetic „ 



Ontogeny is absolutely conditioned by the proper 

 correlation of the stages of these two series at even.' 

 point, and hence it is that any sudden change of en- 

 vironment is usually attended by disastrous conse- 

 quences. Thus, after the fish-like ancestors of air- 

 breathing vertebrates had left the water and become 

 amphibians, they doubtless still had to go back to the 

 water to lay their eggs, in order that the eggs might 

 have the proper conditions for their development. 



Obviously the environment can onlv be altered with 

 e.xtreme slowness, and one of the first duties of the 

 parent is to provide for the developing offspring con- 

 ditions as nearly as possible identical with those 

 under which its own development took place. It is, 

 however, inevitable that, as phylogenetic evolution 

 progresses, the conditions under which the young 

 organism develops should change. In the first place, 

 the mere tendency to acceleration of development, to 

 which we have already referred, must tend to dislo- 

 cate the correlation between the ontogenetic series and 

 the environmental series. Something of this kind 

 seems to have taken place in the life-cycle of many 

 fiydrozoa, resulting in the suppression of the free 

 medusoid generation and the gradual degeneration of 

 the gonophore. But it is probably in most cases 

 change in the environment of the adult that is 

 responsible for such dislocation. 



NO. 2340, VOL. 94] 



To return to the case of the amphibians. At the 

 present day some amphibians, such as the newts and 

 frogs, still lay their eggs in water, while the closely 

 related salamanders retain them in the oviducts until 

 they have developed into highly organised aquatic 

 larvae, or even what is practically the adult condition. 

 Kammerer has shown that the period at which the 

 young are born can be \aried by changing the en- 

 vironment of the parent. In the absence of water 

 the normally aquatic \an2t of the spotted salamander 

 may be retained in the oviduct until they have lost 

 their gills, and they are then born in the fully-de- 

 veloped condition, while, conversely, the alpine sala- 

 mander, of which the young are normally born in the 

 fully-developed state, without gills, may be made to 

 deposit them prematurely in water in the lar\-al, gill- 

 bearing condition. 



There can be no doubt that the ancestral amphibians 

 laid their eggs in water in a completely undeveloped 

 condition. The habit of retaining them in the body 

 during their development must have arisen ver\- 

 gradually in the phylogenetic history of the sala- 

 manders, the period for which the young were retained 

 growing gradually longer and longer. It is obvious 

 that this change of habit involves a corresponding 

 change in the environmental conditions under which 

 the young develop, and in cases in which the young 

 are not born until they have reached practically the 

 adult condition this change directly affects practically 

 the whole ontogeny. We may say that the series 



E] E., E3 E» has become 



Ej' E2' E3' E«' 



and as the change of environment must produce its 

 effect upon the developing organism the series 



M, M., M3 M„ will have become 



Ml' M.; M3' M«' 



We must remember that throughout the whole 

 course of phylogenetic evolution this series is con- 

 stantly lengthening, so that what was the adult con- 

 dition at one time becomes an embryonic stage in 

 future generations, and the series thus represents 

 not only the ontogeny, but also, though in a more or 

 less imperfect manner, the phylogeny of the organism. 

 The character of each stage in ontogeny must 

 depend upon (i) the morphological and physiological 

 constitution of the preceding stage, and (2) the nature 

 of the environment in which development is taking 

 place. We cannot, however, distinguish sharply be- 

 tween those two sets of factors, for, in a certain sense, 

 the environment gradually becomes incorporated in 

 the organism itself as development proceeds, each part 

 contributing to the environment of all the remainder, 

 and the influence of this internal portion of the en- 

 vironment ever becoming more and more important. 

 The whole process of evolution depends upon 

 changes of environment taking place so gradually 

 that the necessarv self-adjustment of the organism at 

 everj' stage is possible. In the case of our amphibia 

 the eggs could possibly undergo the first stages of 

 development, the preliminary segmentation, withm 

 the oviduct of the parent just as well as in the water, 

 for in both cases they would be enclosed in their 

 envelopes, and the morpholc^ical differences between 

 the early stages in the two cases might be expected 

 to be quite insignificant. But it must be the same 

 at each term of the series, for each term is built upon 

 the foundation of the preceding one, and the whole 

 process takes place by slow and imperceptible degrees. 

 It is true that bv the time we reach the formation 

 of the vestigial gill-slits in the embyro of one of the 

 higher vertebrates the environmental conditions are 

 ver\- different from those under which grill-slits were 

 developed in their aquatic ancestors. But what then? 



