Morphology of Organs. 145 



structure that exist between these two divisions of the group 

 Vertebrata. So great are these, that no naturalist of to-day 

 would accept for a moment the view that the mammals are 

 highly developed birds, or that birds have been produced by 

 a great change in the structure of any mammal-like form. 

 Some even go so far as to believe that, while mammals are 

 descended from some amphibian ancestor, the progenitors 

 of birds were reptiles ; but in any case it would probably be 

 held by all that the two groups, if not derived from different 

 classes of reptiles, diverged early and widely from the same 

 type, which must in that case have been a lowly organized 

 form, simple and not highly differentiated. The fact — the 

 unfortunate fact — is that all questions of true homology depend 

 upon what are, for the most part, speculations. It is 

 unquestionably true to say that " to mix up aetiological specu- 

 lations with morphological generalisations" induces confusion 

 into morphology; but it is absolutely necessary to mix up the 

 two, for all that. The test of true affinity must be common 

 descent; and there are, and will be always, divergence of 

 opinion about such matters. It has been, therefore, proposed, 

 and with excellent reason, to change the term "homology" 

 into " homogeny." " Homogeny " signifies community of origin ; 

 and stractures which are clearly derived from each other, or 

 from some common parent form, must be really homogenous. 

 It will be clear to any one that the greatest difficulty of the 

 morphologist is to distinguish between homogeny and homo- 

 plasy; in the case of the two ventricles of the bird's and 

 mammal's heart impossible — ^at present, at least. 



It must, therefore, be carefully borne in mind by the 

 student who reads the following pages, that, while the facts 

 are, it is hoped, correct, the comparisons may be by no means 

 so correct. 



The Body-wall (Epidermis, Dermis, and Muscular 

 Layers). 



In the earthworm the body-wall is divisible into the three 

 layers which have been described ; there is an epidermis 



L 



