THE PRESENT POSITION OF THE THEORY OF ORGANIC EVOLUTION. 109 



[f this view as to the manner in which heredity can be 

 altered is correct, not only does the past history of life as 

 exhibited by fossils become clear to us, but an explanation is 

 afforded of the recapitulation of ancestral history given by 

 embryonic and larval development. We can see that species of 

 animals have become modified in the majority of cases through 

 their entry into a new environment. This entry has usually 

 taken place when the animal has reached the adolescent stage 

 of development, and its structure is then modified as a reaction 

 to the new environment. This modification enables it to exist 

 in the new environment. Its life under the old conditions up 

 to the period of migration constitutes the larval stage of its life- 

 history. As time goes on the reaction to the new environment 

 comes quicker and quicker and finally appears before the 

 migration, and the larval stage is correspondingly shortened. 



Our final conclusion, therefore, is that the laws discovered by 

 Mendel throw no light whatever on the origin of variations, i.e., 

 changes in hereditary potentiality ; they merely show us what 

 will happen if two races already diverse from one another are 

 crossed. But the real problem of biology is the origin of this 

 diversity. 



If the line of reasoning outlined above be sound, it will be 

 gathered that the main position of Darwinism is entirely 

 unaffected by recent discoveries. It is probable that Darwin 

 laid too much stress on the parallelism of the differences 

 between parent wild species and domesticated breed, and 

 those between wild species and wild species. We now know 

 that many of the differences in colour, etc., which distinguish 

 breeds from parent species are pathological differences due to 

 the elimination of a Mendelian factor, and are quite distinct 

 from differences in general proportions due to functional 

 reaction which divide wild species. 



Still, when we recollect that in domestication a species is 

 protected from danger and relieved from the necessity of violent 

 exertion, one cannot help feeling that increase in bulk which so 

 often characterizes it is due to a functional reaction, especially 

 as it has been a matter of slow acquisition, and has not been 

 acquired at a single bound, as we should expect in the case 

 of a quality due to the presence or absence of a Mendelian 

 factor. 



Darwin was most probably mistaken in assuming that the 

 differences in proportion of limbs, etc., which occur between 

 members of the same brood are inheritable. The work of 

 Johannsen and Agar on pure lines seems to show that they are 



