42 EVOLUTION 



seems almost to go out of its way to reveal the 

 evolution of the race. The familiar develop- 

 ment 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 palaeontology 

 and comparative anatomy. 



Following the historical order, we pass from 

 the distributional evidences of evolution 

 whether horizontal and geographical, or 

 vertical and palaeontographical to the ana- 

 tomical data. These are of three kinds, at 

 least : (1) there is the recognition of homo- 

 logies, 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 struc- 

 tures, of which there is no feasible interpreta- 

 tion 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 

 resemble one another in having a similar use, 



