234 PRINCIPLES OF ANIMAL BIOLOGY 



Origin of Homology. — Homology is no accident. The discussion in 

 the chapters on taxonomy and evolution may be anticipated by the 

 stateuKMit that homology results from the inheritance of a common ances- 

 tral condition. All animals now possessing homologous organs nmst have 

 descended from a common ancestral form. The organs of the present- 

 day descendants of that ancestor arise in the same way in the embryo, 

 because in some fundamental respect the organs of the ancestor arose in 

 that manner. If homology is not an inheritance from a common an- 

 cestor, it can hardly have any significance at all. 



If this view is correct, the vertebrates are all related to one another 

 through common ancestry. Birds, and reptiles, and mammals, and 

 fishes, and frogs are all actually distant cousins of one another. As 

 pointed out in Chapter XII, homology is for this reason the chief basis 

 of classification of animals. 



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

 mon descent led, a few decades back, 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, the embryonic stages of animals of today represent the coridition 

 of successive ancestors of these animals. That is, early embrj'onic 

 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 simpl}' bj' studying its individual 

 development. In some cases, this simple procedure is almost feasible. 

 A series of fossil cephalopods (allies of the cuttlefishes), described in 

 Chapter XV, 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. 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 

 highl}' complex types passed through very similar stages, in which the 

 partitions were first straight, then bent, crooked, and finally complicated. 



In most animals, however, embryonic development has undergone a 

 good many changes, so that steps in embryology no longer represent 

 accurately the steps in the evolution of their ancestors. That is, the 

 biogenetic law is less generally applicable than it was formerly supposed 

 to be. However, many important facts of evolution, of limited scope, 

 have been discovered by an appeal to this law. A case in which the 



