THE CONTINUITY OF LIFE 25 



only through a secondary adaptation by which the embryonic 

 and vitelline (yolk) portions of the egg become nearly sep- 

 arated from one another, the connection being retained through 

 a narrow stalk. 



It will be seen by the above exposition of the two historical 

 records, phylogenetic and ontogenetic, that they are by no 

 means complete and that the fragments that exist are often 

 difficult to interpret. This has necessarily occasioned a large 

 amount of controversy among morphologists, not alone in 

 the interpretation of the facts, but even in some cases in the 

 recognition of the facts themselves, owing to the great me- 

 chanical difficulties in the way of their examination. As in 

 all earnest investigation, however, the differences grow less 

 as the work progresses, and at the present time there is a prac- 

 tical agreement upon the main features of vertebrate history, 

 the differences being confined mainly to details. In some 

 cases in the following chapters attempts have been made to 

 set forth divergent views, but, for the most part, both for the 

 sake of clearness and in order to present the matter within 

 suitable limits, the selection has been made of that theory 

 which, in the judgment of the writer, possesses the greatest 

 probability. 



The significance of an anatomical fact depends upon the 

 phylogenetic position of the animal studied, yet at the same 

 time it must be remembered that the only criterion we possess 

 for making the phylogenetic arrangement is that of the an- 

 atomical structure, so that the two lines of investigation are 

 mutually dependent and are likely to become equally modified 

 by the presentation of each new fact. As a basis for this 

 history of the human body, which is at the same time a history 

 of vertebrates, especially of those that lie in the direct line of 

 human ancestry, it is thus necessary to consider the various 

 vertebrate groups, both living and extinct, so far as we know 

 them, and study their mutual relationships as deduced from 

 their structure and development. This is, in fact, a brief study 

 of vertebrate phylogenesis, and will be considered in the next 

 chapter. 



