XV THE PHILOSOPHY OF ZOOLOGY 645 



the varieties presenting these characters are accordingly spoken of 

 respectively as either dominant or recessive. When the hybrids, all 

 showing the dominant character, produce a second generation by 

 self-fertilisation, or fertilisation among themselves, there is a re- 

 appearance of the recessive character in a certain fixed proportion 

 of the progeny. In this second generation one quarter are pure 

 recessives, one quarter pure dominants, and the rest of mixed char- 

 acter. The pure recessives in following (inbred) generations always 

 remain true to the recessive character the dominant having 

 evidently become eliminated from their constitution ; and the 

 same holds good, mutatis mutandis, for the dominants. The 

 intermediate forms present the dominant character, but when 

 inbred they behave exactly like the original hybrid, that is to say, 

 the progeny consist of pure dominants (a quarter), pure recessives 

 (a quarter), and mixed or intermediates (a half). In future genera- 

 tions these proportions are regularly maintained. 



It is inferred from their behaviour in inheritance that the 

 Mendelian characters occur in pairs, the members of which have a 

 fixed reciprocal relationship to one another, one of them being 

 usually dominant, as in the case of tallness and shortness in the first 

 example referred to : two such characters are said to be allelo- 

 morphic, or reciprocating, with regard to one another. The term 

 segregation is applied to the process by which each becomes separated 

 out in the pure dominants and pure recessives of the second and 

 succeeding generations. 



An important point in connection with the bearing of Mendelian 

 inheritance in plants and animals on the problems of evolution is 

 that the former provides a means by which new variations or 

 mutations when they arise may be at once fixed and become 

 perpetuated. 



Orthogenesis. The fact that, as shown by the evidence afforded 

 by both existing and extinct forms, organisms vary in such a way 

 as to follow definite lines leading to special adaptations (and 

 sometimes to excessive development of parts) is held not to be 

 explainable by a theory of selection of slight variations the direction 

 of which is under no known guidance. This definiteness in the 

 direction of evolution orthogenesis is a fact in nature of which 

 there is evidence on all sides. In nearly all groups and nearly 

 all systems of organs there is evidence of progressive development 

 such as cannot be supposed to be due simply to the fortuitous appear- 

 ance and selection of the Darwinian variations. A nervous system, 

 simple, diffused, and superficial in the lower members of a group, 

 becomes more highly elaborated, more complex, more deeply placed, 

 in the higher. Light-perceiving organs, mere groups of pigmented 

 elements, and nerve-cells with refractive bodies, in the lower forms, 

 become represented in the higher by complex eyes with elaborate 

 mechanism for the reception of images of external objects. Appen- 



