i6z 



THE QUARTERLY REVIEW OF BIOLOGY 



differentiate. If in a larval amphibian 

 the skin overlying the optic vesicle is 

 removed and replaced by skin from any 

 other part of the body, a perfect lens is 

 formed by the transplanted skin (W. H. 

 Lewis). Furthermore an optic vesicle, 

 grafted into an abnormal site, proceeds to 

 modify the overlying skin so that it 

 becomes a lens. In this case the influence 

 exerted at close range by the optic vesicle 

 upon the indifferent skin tissue is not 

 simply that of a chemical messenger or 

 hormone; something more suggestive of 

 physical moulding must come into play, 

 otherwise it is hard to conceive how the 

 proper amount of (indifferent) tissue is 

 selected and adjusted to fit the mouth of 

 the vesicle. 



While we have merely touched on the 

 fringe of this more modern line of investi- 

 gation, the instances given suffice to show 

 that the moulding influence of the internal 

 environment of an Anlage that is not 

 self-differentiating has been brought with- 

 in the scope of experimental study. In 

 our selected cases the decisive influence 

 exerted upon the non-self-differentiating 

 Anlage has been proved to have its seat 

 in one single primary or controlling organ 

 (or Anlage), to which the other is second- 

 ary or subservient — in which respect 

 Lankester's supposition of plasis is rendered 

 less vague, more definite. Even in cases 

 of this order of simplicity, where, during 

 ontogeny, we see a direct and all-powerful 

 reaction of one single organ upon another, 

 the physical means of interaction between 

 the pair of correlated organs is not the 

 same in every case. This must be kept in 

 mind when we come to deal with the 

 exceedingly difficult cases of phylogenetic 

 moulding or adaptation. 



spemann's criticisms of the homology 

 conception 



In a special article on the vexed subject 

 of homology Spemann (191 5) has utilized 



some of these experimental results to 

 criticise not only the conception of 

 homology in itself but also, apparently, 

 Lankester's distinction between homog- 

 eny and homoplasy. 



Citing his own experiments and those of 

 Lewis on development of the lens, he 

 points out that the lenses in the experi- 

 mental cases are not homologous with 

 normal lenses; nor are they homogenetic; 

 they are, however, homoplastic. But, 

 he says, the experiments also lead inevi- 

 tably to the conclusion that lenses formed 

 in normal development are both homo- 

 genetic and homoplastic, whereupon he 

 feels constrained to ask whether Lan- 

 kester's distinction has any deeper mean- 

 ing. 



This obscure remark seems to mean one 

 or other of two things. Either Spemann 

 has failed to understand Lankester, or, 

 reflecting on the bearing of his own 

 experiments, he has been momentarily 

 waylaid into a pessimistic, cut bono! 

 frame of mind. We can hardly believe 

 the latter, i.e., we can scarcely suppose 

 that he doubts the fact of homogeny 

 (he does not doubt homoplasy). The 

 valid examples of homogeny are the group 

 features common to two or more great 

 assemblages of animals by which is 

 determined their subordination under a 

 common category. It is the steady and 

 extraordinary persistence of structural 

 similarity under all apparent disguises, 

 under all variety of environment and 

 adaptation, that created the science of 

 morphology to begin with, and that later 

 provided the chief argument for common 

 descent and furnished the means for a 

 natural classification of animals. Homog- 

 eny with its very specific implications is 

 not a thing of moonshine. We are left 

 to suppose that Spemann has simply 

 misread his authority. Now, Lankester 

 has well foreseen that homoplasy may in 

 some cases coincide with homogeny, as 



