PRINCIPLES OF VERTEBRATE MORPHOLOGY 23 



fossils. These three phenomena form the chief subject-matter of 

 the sciences of comparative anatomy, embryology, and palaeontology. 



HomolcTgies have already been dealt with incidentally in the fore- 

 going discussion of adaptations; for the second law of adaptation, 

 "the law of divergence of form," implies homology, since the variously 

 modified adaptive structures are the products of a single generalized 

 ancestral prototype, and are strictly comparable in fundamental 

 structure and mode of origin. Thus the wing of a bird, the fore leg 

 of a horse, and the flipper of a whale are homologous in spite of their 

 profound departures, both structurally and functionally, from the 

 generalized vertebrate fore limb. Similarly, the rudimentary hind 

 limbs of a whale and the splint bones of a horse are recognized as 

 homologues of the fully functional structures of allied groups. 



We must be constantly on our guard against the all too common 

 error of mistaking for homologies " convergences of form" such as 

 are dealt with in Osborn's first law of adaptation. Thus, while the 

 shark, ichthyosaur, and porpoise appear to possess homologous 

 adaptations in their general form and locomotor organs, such struc- 

 tures as the caudal and dorsal fins in these three types are more truly 

 analogous than homologous, for they do not represent the same em- 

 bryonic rudiments, nor are they made of the same structural materials. 

 Before we can be certain about homologies we must put them to the 

 test by means of a study of their embryonic development. 



EMBRYOLOGICAL ASPECTS OF PHYLOGENY 

 THE RECAPITULATION THEORY 



This theory has often been called Haeckel's Biogenetic Law, and 

 may be paraphrased as follows: The life history of the individual 

 gives a brief condensed resume of the evolutionary history of its an- 

 cestors. Some one has said that "the individual climbs its own 

 ancestral tree." In a somewhat restricted and modified sense this 

 theory is still accepted by most embryologists, though there are some 

 who have cast it aside as worthless. Embryology has been compared 

 to "an ancient manuscript with many sheets lost, other displaced, 

 and with spurious passages interpolated by a later hand." The fact 

 that the history is tremendously abbreviated precludes the recapitula- 

 tion of every ancestral stage. The necessity for the embryo to prepare 

 quickly to meet a difficult environment and to obtain its own liveli- 



