PRINCIPLES OF TAXONOMY 255 



do not have gills in the adult. Thus, on the whole, animals whose 

 adult structure is similar resemble each other even more closely in 

 embryonic stages. Similarity of embryos is particularly useful in taxon- 

 omy in those instances in which the adult animals, though closely related, 

 have become so changed as to lose all similarity. An example of this 

 kind is found in the parasite Sacculina described in Chap. 23. Biolo- 

 gists believe that similarity of structures in the embryo can be due only 

 to similarity of the evolution of those structures ; and because resemblance 

 in the embryo sometimes remains after adult similarity has been dimin- 

 ished or destroyed, embryonic development is frequently better evidence 

 of homology than is adult structure of the same animals. 



The only known phenomenon which could preserve the similarities 

 possessed by different animals is heredity. The likenesses of present- 

 day animals must therefore be inherited from like animals of the past. 

 Since it is scarcely conceivable that two identical organisms ever could 

 have arisen independently of one another, inheritance from like ancestors 

 must ultimately be inheritance from the same ancestors. Animals of 

 different modern groups are held to possess like features in both adult 

 and embryo because of this descent from a common source. This is the 

 argument upon which the taxonomist relies when he classifies animals 

 on the basis of supposed homologies. 



Biogenetic Law. — The evident dependence of homology upon a com- 

 mon descent led, in the last century, to a conception comprised under 

 the term biogenetic law, sometimes called by the more expressive and 

 less committal name recapitulation theory. According to this law or 

 theory (already stated page 74), the embryonic or other early stages of 

 individual animals of today represent the condition of successive ancestors 

 of these animals. That is, early developmental conditions represent 

 very remote ancestors, later embryonic stages represent more recent 

 ancestors. Some biologists held that the early embryonic stages are like 

 the adult ancestors; others believed merely that the embryonic stages 

 of the present are like the embryonic stages of the ancestors. 



If this law were capable of rigid application, it would be easy to trace 

 the evolutionary history of a race simply by studying the development of 

 its individuals. In some cases this simple precedure is almost feasible. 

 A series of fossil cephalopods (allies of the cuttlefishes) is a case in point. 

 The fossil remains of these animals indicate that, in their racial history, 

 their shells were at first provided with straight partitions, later with 

 partitions whose edges were bent, crooked, and finally lobed in a very 

 complicated manner (Fig. 210). Since in the fossils both the young and 

 old stages of each individual shell are preserved, it is possible to compare 

 the individual development with the racial development. When this is 

 done, it appears that the individuals of the highly complex types passed 



