Ortmann.] IJU [May 15, 



certain sum of conditions of life are destroyed. The remnant left is fit 

 for existence, and all the individuals surviving are able to live and propa- 

 gate. There may be slight differences between them, especially as regards 

 characters not bearing on utility, but a certain average of good characters 

 is present. Natural selection at least preserves this good average, and if 

 there arise any useful characters, a smaller percentage of the individuals 

 possessing the latter is destroyed, and thus the better individuals may 

 gain little by little the preponderance in number : the average is displaced 

 slowly in a distinct direction, namely, toward the better. This latter 

 "mutation" is distinguished from the mutation by orthogenesis by the 

 advantage connected with the particular line in which the change 

 advances. Natural selection effects a general adaptation of the whole 

 number of the surviving individuals to particular conditions of life. 



4. But natural selection does not form species ; it only preserves or 

 transforms already existing species. If we suppose, however, that of tlie 

 individuals surviving in natural selection difterent groups are separated 

 from each other under different conditions, and that this separation 

 cannot be overcome, so that each group must remain under the constant 

 action of particular conditions, the difference of the latter effects, that 

 each group tends to develop its characters in a different direction. It is 

 true, if upon each separated group the same external conditions act in the 

 same manner, there would be, of course, no separation of the directions 

 of development. But differentiation of the external conditions by bio- 

 nomic separation, and the splitting into groups of individuals living for- 

 merly under the same conditions wiU give origin to different characters in 

 each group, and animals distinguished by the constant presence of differ- 

 ent characters we call species. Different species are formed hy bionomie 

 separation ; separation does not always imply differentiation of the condi- 

 tions of life, and accordingly does not always form new species ; but if there 

 is a differentiation into species, it is always due to separation under different 

 bionomie conditions. 



In the above the particular action of each of the four chief factors play- 

 ing a part in the evolution and diversification of the organic world is 

 properly limited. We have seen that the two last-named factors, selec- 

 tion and separation, are imitated by man in the breeding of domesticated 

 animals. Both nature and man use the material furnished by variation, 

 and the success of both is warranted under the condition that the acquired 

 characters maj^ be fixed by hereditary transmission. The four factors 

 named, variation, inheritance, selection and separation, must work 

 together, in order to obtain different species, and, indeed, they do so 

 always; it is impossible to think that one of them should work by itself, 

 or that one could be left aside. 



The proper action of each of these factors was recognized almost cor- 

 rectly by Darwin, only as respects the <lifterentiation of species, which he 

 attributes to the principle of divergence, he was not quite satisfied.* But 



* Darwin, Origin, p. 87 : " Though it was a long time before I saw how." 



