EVIDENCES OF EVOLUTION 41 



(c) It rests thirdly upon embryological 

 evidence, for the individual development 

 seems almost to go out of its way to reveal 

 the evolution of the race. The familiar de- 

 velopment of frog-spawn into tadpoles and 

 froglings is in some respects almost startling 

 in its recapitulation of the evolution of the 

 Amphibian race from fish ancestors — an evo- 

 lution vouched for by the data of palaeon- 

 tology and comparative anatomy. 



Following the historical order, we pass 

 from the distributional evidences of evo- 

 lution — whether horizontal and geographical, 

 or vertical and palseontographical — to the 

 anatomical data. These are of three kinds 

 at least: (1) there is the recognition of 

 homologies, i. e. of deeply-rooted structural 

 and developmental similarities; (2) there are 

 the facts of classification, that species fades 

 into species, that genus is linked to genus, 

 that tentative genealogical trees are possible; 

 and (3) there is the occurrence of vestigial 

 structures, of which there is no feasible in- 

 terpretation except in terms of past history. 



Homologies. — When two or more struc- 

 tures, organs or specialized parts, in one and 

 the same organism, or in several organisms, 

 show a deep resemblance in their architecture 

 and also in their manner of development, 

 they are said to be homologous. When they 



