290 THE EVOLUTIONARY CAREER 



Consider how this developmental change is manifested : when 

 transformism occurs the tectonics of the development undergoes 

 change — hut this change is not a radical one. Thus the axial 

 skeleton of the cyclostomes is a notochord, or undivided skeletal 

 rod with a continuous sheath, but the axial skeleton of a Teleost 

 fish is a backbone consisting of articulating segments, or vertebrae. 

 In the development of the vertebral column of a Teleostean fish 

 the notochord is not, however, absent : it develops as if the 

 definitive axial skeleton were going to be notochordal and then 

 the cells of the mesoblastic somites proceed to form the vertebral 

 rings. By and by the notochord becomes a mere vestige. So also 

 the water-breathing animal acquires gills or branchiae : these 

 develop as pharyngeal clefts that become provided with a series 

 of bony arches and gill-filaments. Now when the ovum that is 

 going to become an air-breathing animal develops and becomes 

 provided with lungs this branchial apparatus is still there although 

 it never becomes functional. Nor do the lungs arise develop- 

 mentally de novo : they are developed from the swim-bladder, 

 which never becomes functional in an air-breathing animal. 

 And the larval anlagen of the branchiae become developed into 

 the skeleton of the tongue, the Eustachian tube, some of the bones 

 of the middle ear, etc. 



Thus when transformism occurs the developmental tectonics 

 become changed. New functions are performed by organs that 

 develop from the anlagen of organs that are being superseded — 

 in the functional sense. 



And so ontogenies may be expected to record transformist 

 events — to some extent, at least, and, therefore, homologies — 

 which are based on developmental events (or ontogenies) must 

 record (it may be imperfectly) these transformist events, and 

 consequently animal affinities. 



96^. The Conception of Recapitulation. This was that 

 an individual development recapitulated, in abbreviated and 

 distorted fashion, the evolution of the race to which the individual 

 belonged : ontogeny was said to repeat phylogeny. A mammal, 

 for instance, w^as regarded as exhibiting a piscine phase in its 

 development — indicating that living and functional piscine 

 animals were in its ancestry. 



It is easy to demonstrate the crudity of the conception — when 

 stated as above. There are embryonic structures in the mam- 



