606 ZOOLOGY SECT. 



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 character. 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 generations these proportions are regularly 

 maintained. 



Such is the nature of the fundamental experiments of the 

 Mendelian system. For further developments, the details of cases 

 among animals, of cases of the crossing of varieties differing from 

 one another in more than one character, of cases in which a blend- 

 ing of the parental characters appears in a proportion of the 

 progeny, &c., as well as the practical applications in stock-raising 

 and horticulture, reference may be made to the works mentioned 

 in the Appendix. 



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 some- 

 times 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 defmiteness 

 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 de- 

 velopment such as cannot be supposed to be due simply to the 

 fortuitious appearance 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. Appendages undergo progressive modification, 

 so that each pair becomes specially adapted for the performance of 

 particular functions. Moreover, parallel and evidently independent 

 lines of orthogenetic development are in many cases traceable in 

 separate groups. As examples may be mentioned the series of 

 stages in the development of .complex eyes from simple rudiments, 

 which are observable in the Annulata, the Arthropoda and the 

 Chordata : the parallel and quite unconnected stages in the reduc- 

 tion of the digits, leading to the greater perfection of the limbs as 

 running organs, to be traced in the Perissodactyle and Artiodactyle 



