138 GENEPwAL EYOLUTIOK 



quainted. And inasmucli as almost all groups, as genera, orders, 

 etc., which are held to be distinct, but adjacent, present certain 

 points of approximation to each other, the almost daily discovery 

 of intermediate forms gives us confidence to believe that the 

 pointings in other cases will also be realized. 



y. Of Transitions. 



The preceding statements were necessary to the comprehension 

 of the supposed mode of metamorphosis or development of the 

 various types of living beings, or, in other words, of the single 

 structural features which define them. 



As it is evident that the groups of highest rank have had their 

 origin in remote ages, cases of transition from one to the other by 

 change of character can not be witnessed 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 kingdom (e. g., Verte- 

 brate, Mollusc, etc.) are quite identical in structural character at 

 their first appearance on the germinal layer of the yolk of the 

 parent Qgg. It shows that the character of the respective groups 

 of high rank api^ear 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 re- 

 lation to each other that we have already seen in the case of those 

 definitive of orders, etc. In a natural 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 ad- 

 vanced stage of that highest. In this manner many stages of the 

 highest genus a2:)pear to be represented by permanent genera in all 

 natural groups. Generally, however, this resmblance 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 gross illustration) a frog at one stage of growth 

 has four legs and a tail ; the salamander always preserves four 

 legs and a tail, thus resembling the young frog. The latter is, 

 however, not a salamander at that time, because, among other 



