648 LAND MAMMALS IN THE WESTERN HEMISPHERE 



it deals only with existing forms and possesses no sure cri- 

 terion for determining the value of similarities. It is thus 

 unable to distinguish with certainty between those resemblances 

 which are due to inheritance from a common ancestry and those 

 which have been independently acquired. It is a very fre- 

 quent fallacy to assume that, because two allied groups, 

 B and C, possess a certain structure, their common ancestor, 

 A, must also have possessed it. This may or may not have 

 been the case, and Comparative Anatomy offers no assured 

 means of deciding between those alternatives or of confidently 

 distinguishing primitive characters from degenerative or retro- 

 grade changes. 



(2) Embryology, which is the study of the development of 

 the individual animal from the unfertilized egg to the adult 

 condition, was long regarded as the infallible test of theoretical 

 views in zoology. This was on the assumption that individual 

 development {ontogeny) is a recapitulation in abbreviated form 

 of the ancestral history (phylogeny) of the species, and was called 

 by Haeckel ''the fundamental biogenetic law." It was soon 

 learned, however, that the "recapitulation theory" was not 

 to be implicitly trusted, for structural features which could 

 not possibly be a part of ancestral history were imposed upon 

 or substituted for those due to phylogenetic inheritance. Now 

 the whole theory is strongly questioned, and the absence of any 

 universally accepted rules of interpretation, by which the con- 

 tradictory embryological data may be harmonized into a con- 

 sistent whole, has deprived the method of that authoritative 

 character once so generally ascribed to it. It is like dealing 

 with a literature which has been vitiated with many forgeries, 

 only the grossest of which can be readily detected. Embryol- 

 ogy has rendered many great services in the solution of 

 zoological problems and will no doubt render many more, 

 but it cannot, of itself, reach final conclusions. 



(3) Experimental Zoology, especially that part known as 

 ''Genetics," one of the newest and most promising provinces 



