16* ( i8 ) 



had their origin in remote ages, cases of transition from 

 one to the other by change of character cannot be wit- 

 nessed at the present day. We therefore look to the 

 most nearly related divisions, or those of the lowest 

 rank, for evidence of such change. 



It is necessary to premise that embryology teaches 

 that all the species of a given branch of the animal king- 

 dom (e. g. y Vertebrate, Mollusc, etc.) are quite identical 

 in structural character at their first appearance on the 

 germinal layer of the yolk of the parent egg. It shows 

 that the character of the respective groups of high rank 

 appear first, then those of less grade, and last of all 

 those structures which distinguish them as genera. But 

 among the earliest characters which appear are those of 

 the species, and some of those of the individual. 



We find the characters of different genera to bear the 

 same relation to each other that we have already seen 

 in the case of those definitive of orders, etc. In a natu- 

 ral assemblage of related genera we discover that some 

 are defined by characters found only in the embryonic 

 stages of others ; while a second will present a perma- 

 nent condition of its definitive part, which marks a more 

 advanced stage of that highest. In this manner many 

 stages of the highest genus appear to be represented by 

 permanent genera in all natural groups. Generally, 

 however, this resemblance does not involve, an entire 

 identity, there being some other immaturities found in 

 the highest genus at the time it presents the character 

 preserved in permanency by the lower, which the lower 

 loses. Thus (to use a very coarse example) a frog at 

 one stage of growth has four legs and a tail : the sala- 

 mander always preserves four legs and a tail, thus re- 

 sembling the young frog. The latter is, however, not a 



