BDELLOSTOMA DOMBEYI, LAC. 149 



Thus it is that the evidence in favor of progressive simplifi- 

 cation of structure (which is all that is meant by the term 

 degeneracy) depend upon two factors : 



(a) The comparison of the anatomy of the fully developed 

 degenerate animal with that of its supposed congeners. In 

 this way we can often be sure that the less complex animal 

 does not represent any of the ancestral stages common to its 

 nearest relatives. This is the anatomical method, and (b) by 

 the study of the embryological development of the forms under 

 consideration. This is the embryological method, and it is very 

 generally admitted to be conclusive whenever it can be shown 

 that either the embryonic or larval stages of the animal 

 under consideration possess a higher degree of morphological 

 differentiation than is possessed by the adult form. e. g. 

 barnacles (Cirrhipedes), Tunicates. In many instances the 

 embryological history may be, and is, so shortened as to give no 

 trace of a higher stage of existence having ever been enjoyed. 

 e.g. many Tunicates. Even from this very short review of 

 the degeneration hypothesis, it is evident that we must 

 conclude that a priori there is much reason to suppose that 

 any given animal form may have developed from a more highly 

 organized ancestor by a process of simplification as to assume 

 that it has developed from a relatively simpler form by an 

 unbroken process of complication, and that our only means 

 outside of morphological research is to be derived solely from 

 the life habits of the animal, i.e. whether they are such as to 

 favor degeneracy or not. In saying that there is much reason, 

 on a priori grounds, to assume that an animal has degenerated 

 or developed downward to its present condition as that it has 

 developed upward, I certainly cannot acquiese in the view 

 expressed by Dohrn and adopted by Lankester that there is as 

 much probability in favor of the one process as of the other, 

 for it is evident that cases of degeneracy must be much less 

 common than cases of progression, otherwise the Law of 

 Agassiz, fully established by Darwin, would not be so easy 

 of observation in embryology and palaeontology. The law of 

 progressive differentiation is so universal in its application that 

 we are on safe ground in always considering any existing form 



