58 THE EVOLUTIONARY THEORY 



Darwin based his theory upon the accumulation of the 

 sHght variations that are constantly occurring through- 

 out the organic world, and are called "normal" and 

 ''continuous." ^ These are often very sHght indeed, 

 but Darwin beheved that they afford a sufficient basis 

 for the operation of the principle of natural selection. 

 The slightest difference between organisms, he main- 

 tained, is often sufficient to determine a survival of one 

 variety and an extinction of another. 



The fact of heredity comes in at this point. Favour- 

 able variations are preserved in offspring, and become 

 gradually accentuated, accumulating from generation 

 to generation until the divergence from parental forms 

 becomes sufficient to constitute a new and independent 

 species. The obvious objection has been made that, 

 under the conditions of unrestrained nature, this accu- 

 mulation of variations is prevented by constant cross- 

 breeding between the members of the same species. 

 Accordingly Darwin hypothecated the incidental factor 

 of isolation, as supplementing natural selection by help- 

 ing to preserve the variations which afford the basis 

 of the working of that principle. This isolation, or 

 segregation as it is also called, is said to be geographical 

 when caused by migration, and biologic when due to 

 the deterring effect of variations upon miscellaneous 

 inter-breeding.2 



1 On continuous and discontinuous variations, see R. H. Lock, 

 Recent Progress, pp. 14-18. The mutations theory is based upon 

 discontinuous variations. 



"^Origin of Species, Vol. I. pp. 127-132. Cf. M. M. Metcalf, 

 Organic Evolution, pp. 60-67. Darwin also found it necessary to 



