isms that have evolved along definite lines, and where the 

 variation has apparently been predetermined. In some in- 

 stances these variations developed to such an extent that 

 they became disadvantageous, for example, the unwieldy 

 Cretaceous reptiles, the coils of the Ammonites, the tusks 

 of the mammoth, the giant antlers of the Irish stag, and the 

 sabre-toothed tigers, among extinct animals. 



Coulter says: "The history of such a group as Gymno- 

 sperms shows a tendency to vary in certain definite direc- 

 tions that have persisted from the early Paleozoic to the 

 present time." We also see Orthogenesis probably in the 

 cdd pinnateness of the leaves of the sumacs, and in the 

 abrupt pinnateness of the leaves of the partridge-pea, etc. 

 There is a "racial tendency toward some particular line of 

 development." Overspecialization can be explained by 

 this theory — for example, the tusks of the wild boar, the 

 giant horns of many wild sheep and goats, the enormous 

 beaks of several rhynchophorous beetles, etc. 



The explanation of this persistent variation still eludes 

 us. Nageli (1884) explained Orthogenesis by means of 

 "an inner directive force," an inherent force in the organic 

 world that makes for progressive development. ^ Eimer 

 (1888) on the other hand believed that Orthogenesis is 

 produced and controlled by external factors of climate, 

 food supply, and environment generally. Many biologists 

 believe that Natural Selection, Mutation, and Orthogenesis 

 may all be operative in producing new forms (Kellogg, 

 Darwinism To-day). 



Discussion: — Does Natural Selection play any part in 

 Orthogenesis? 



(f) — Protoplasm and the Origin of Life 



Huxley very properly and aptly termed protoplasm 

 "the physical basis of life" for everywhere life is associated 

 with protoplasm. Evolutionists, therefore, in seeking for 

 the origin of life have as their fundamental problem the 

 origin of protoplasm. According to the mechanistic 

 school, the problem is one belonging wholly to physics and 

 chemistry, but the vitalistic school sees the working of a 

 directive force, termed vitalism, in connection with the 

 physico-chemical changes. 



In the present state of science the solution of the pro- 

 blem should not be expected. Our knowledge of pro- 

 toplasm is very incomplete on account of its organic com- 



(i)_Contrast this theory with that of Aristotle's "perfecting principle." 



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