310 COMPARATIVE ANATOMY 



common to all the Chordata. The same may be said of 

 other organs, such as the excretory tubules. 



A plan may be constructed on similar principles for the 

 Arthropoda, another for the Mollusca, and so forth, and 

 the greater the number of animals that one studies the more 

 one is impressed with the fact that each is built upon a par- 

 ticular plan characteristic of the phylum to which it belongs. 



It is the first business of the comparative anatomist to 

 convince himself that the various animals included in a 

 single phylum are really constructed upon a common plan, 

 and to form as clear a conception as possible of that plan. 

 Having done this he may ask himself the question, Why 

 are all these animals constructed after the same pattern? 

 And further, Why do the embryos of higher vertebrates 

 develop organs resembling those which are permanent in 

 lower vertebrates, only to lose them again before their 

 development is completed? 



There is only one satisfactory answer to these questions 

 viz. that the similarity of plan, the likeness, is due to blood- 

 relationship, and the embryonic characters of the higher 

 forms are the relics, the reminiscence, if one may use the 

 term, of the structure of some remote ancestor whose 

 organisation had not advanced beyond the stage of com- 

 plexity now exhibited by the lower members of the phylum 

 in question. 



It has been the object of these volumes to show that 

 there are good grounds for the belief that this explanation 

 of the phenomena of anatomy and embryology is a true 

 one; that the animals comprised in a phlyum are what 

 they are because they have diverged in various directions 

 and to various amounts from the structure of a common 

 ancestor, while retaining the main features of that structure. 



But it must be understood that this explanation cannot be 

 proved to be true ; it can only be shown that the evidence in 

 its favour is so cogent that no other explanation is possible. 

 Naturally, one cannot bring forward a sufficiently overwhelm- 

 ing mass of evidence from the study of a few selected examples. 

 But the student who has mastered the contents of these 

 volumes should have no difficulty in extending his studies to 

 other animals, until he has accumulated an amount of evidence 

 that is practically irrefutable. 



