152 MEANING OF THE SYSTEM. 



The impossibility of a sharply defined classification is also rendered 

 comprehensible by the theory of descent. The theory requires the 

 existence of forms transitional between intimately and remotely 

 allied groups ; and explains, as a result of the disappearance, in course 

 of time, of numerous types which have been worsted in the struggle 

 for existence, the fact that groups of equal value are of such various 

 extent, and are often only represented by single forms. 



It is not only systematic characters, but also the innumerable 

 facts brought to light by the science of Comparative Anatomy which 

 point to a nearer or more remote relationship between the different 

 groups. For example, if we examine the structure of the extremities 

 or the brain of Vertebrates, we find, in spite of considerable differ- 

 ences (sometimes bridged over by intermediate forms) in the various 

 groups, that in all they are built upon a common type of struc- 

 ture. This type is found very variously modified and more or 

 less differentiated in each secondary group, according to the different 

 functions which the organ has to fulfil and according to the exigencies 

 of the mode of life to which each species is subjected. In the fin of 

 the whale, in the wing of the bird, in the anterior limb of the 

 quadruped, and in the human arm it can be shown that there are 

 present the same bones, here short and broad and irnrnoveably con- 

 nected, there elongated and jointed in different ways to allow of 

 corresponding movements, sometimes with every part fully developed, 

 sometimes simplified in one way or another, and partly or entirely 

 rudimentary. 



Evidence from the facts of Dimorphism and Polymorphism. 

 The phenomena of dimorphism and polymorphism in the same 

 species, and the sexual differences which have been developed in 

 animals originally hermaphrodite, may be quoted as important evi- 

 dence of the extensive influence of adaptation. 



Male and female forms differ not only in the fact that the former 

 produce spermatozoa and the latter ova, but they exhibit numerous 

 secondary sexual characteristics connected with the different func- 

 tions which the male and female respectively have to perform. The 

 existence of these secondary characteristics can in all cases be 

 satisfactorily explained by means of natural selection. We may 

 therefore, in a certain sense, speak of a sexual selection * by means 

 of which the two sexes have been, in course of time, gradually sepa- 

 rated from one another, not only in peculiarities of form and organiza- 



* Ch. Darwin, "The Descent of Man, and. Selection in Relation to Sex," 

 Vol. I. and II. London 1871. 



