BASIS OF THE PROBLEM 79 



as are many of the details connected with the process, there is 

 a sufficient measure of agreement regarding the main facts to 

 enable an answer shortly to be formulated. The answer rests 

 upon two series of facts. The observed characters of organisms 

 are based upon certain predispositions in the germinal con- 

 stitution. These predispositions or factors under the stimuli of 

 the environment give rise to the various characters. These factors 

 are derived from both parents through the fusion of the gametes, 

 and the complex of factors in the offspring is the result of a chance 

 mixture of factors. There may further at times be a dropping 

 out, an addition to, and perhaps a modification of the factors. 

 Secondly, the death-rate is selective. Although the result of 

 selection is confused by the effect of modifications which smooth 

 over the differences due to differences in the germinal constitution, 

 the effect is that individuals with certain factors are favoured 

 and others with different factors are eliminated. Therefore the 

 average nature of the factors may be changed should any type, 

 other than that approximating to the normal type, be favoured. 

 It is possible in this manner to understand how change has 

 come about. There are many difficulties, but they are all capable 

 of fairly satisfactory explanation. None at least is insuperable. 

 There is one problem which is perhaps somewhat difficult to solve 

 unless a particular supposition is introduced. This problem is 

 connected with the evolution of organs along certain lines, as, for 

 instance, the evolution of horns and teeth in the vertebrates. 

 This class of evidence has chiefly impressed itself upon palaeonto- 

 logists, and one of the most distinguished of them has stated that 

 from this class of evidence he concludes that ' there are funda- 

 mental predispositions to vary in certain directions '.* This is 

 the supposition which it may be necessary to introduce, but which 

 does not in any way conflict with what has been said. 



1 Osborn, Evolution of Mammalian Molar Teeth, p. 228. One of the most 

 valuable contributions to the discussion of orthogenesis has been made by Plate, 

 tfber die Bedeutung des Darwinischen Selectionsprinzips. Plate concludes that 

 the evidence points to some kind of orthogenesis. 



