PHYLOGENY AND ONTOGENY 



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adult, undergo modification during evolution, and that some 

 caenogenetic characters are very much more ancient than others. 

 The relation which exists between the phylogenetic and onto- 

 genetic stages in the evolution of an organism are very diagram- 

 matically expressed in Fig. 131, which is drawn with special 

 reference to the familiar case of the frog. The vertical line is 

 supposed to represent the ancestral history, some of the principal 

 stages of which are indicated. 

 The horizontal line at the top Sc * 9 " ^ 



represents the individual life- 

 history, with the stages indi- 

 cated which represent those 

 selected in the ancestral his- 

 tory. The ovum represents 

 the protozoon ancestor ; the 

 blastula the hollow, spherical 

 protozoon colony; thegastrula 

 the cffilenterate ; the meta- 

 merically segmented embryo 

 the segmented worm, and the 

 tadpole the fish-like stage. 

 The oblique lines represent 

 the lines of evolution of the 

 different ontogenetic stages 

 themselves, in the course of 

 which caenogenetic characters, 

 such as the acquisition of 

 food-yolk by the ovum and of 

 suckers and horny jaws by 

 the tadpole, have arisen. The 

 shorter horizontal lines repre- 



FIG. 131. Diagram illustrating the 

 Eelation between Ontogeny and 

 Phylogeny. 



sent the ontogeny at the different ancestral stages, and their 

 increase in length from below upwards is obviously due to the 

 successive addition of new stages to the ontogenetic record, while 

 the shortness of the horizontal lines as compared with the corre- 

 sponding portions of the vertical line may be taken to indicate, 

 though very imperfectly, the abbreviation of the ontogeny. 



In the vegetable kingdom we find illustrations of the law of 

 recapitulation and of the complication of the ontogenetic record 

 by the development of caenogenetic characters, quite as striking 

 as any which we find amongst animals. Take, for example, the 



