THE PROBLEM OF SPECIES 5 



kind of work have set themselves for solution is that 

 of the nature and method of origin of the existing 

 differences between certain groups of organic beings 

 namely, species. Basing their studies on the doc- 

 trine that the present species have arisen through the 

 modification of pre-existing species, they endeavour 

 to observe how modifications of existing species do 

 actually arise in Nature, as well as under domestica- 

 tion ; and they watch the hereditary transmission of 

 the modified forms when like is. bred with like, and 

 when different types are crossed together. For the 

 theory of uniformity, now universally accepted, teaches 

 us that the organisms with which we are now familiar 

 owe their present characteristics to the accumulation 

 of a series of changes similar to those which are still 

 in progress. It has, therefore, appeared likely to a 

 few that a further understanding of the processes of 

 evolution might best be obtained by a closer study* 

 firstly, of variation, or the ways in which offspring 

 differ from their parents ; and, secondly, of inheritance, 

 or the ways in which the resemblances between parents 

 and their offspring are perpetuated from one genera- 

 tion to another. 



It may be well to point out at once that the further 

 study of the method of origin of new species, admitting, 

 as it does, that this process is not yet by any means 

 fully understood, does not for this reason imply that 

 the theory of organic evolution itself is open to criti- 

 cism. The evidence that new species arise by the 

 modification of pre-existing species is quite indepen- 

 dent of the evidence that this process invariably occurs 



