440 PROGRESS OF SCIENCE IN THE CENTURY. 



the great variety of ways in which, in the course of 

 nature, the range of intercrossing is restricted e.g., 

 by geographical barriers, by differences in habit, by 

 psychical likes and dislikes, by reproductive varia- 

 tion causing mutual sterility between two sections of 

 a species living on a common area, and so on. Ac- 

 cording to Romanes, " without isolation, or the pre- 

 vention of free inter-crossing, organic evolution is 

 in no case possible." Again it has to be confessed 

 that the body of facts in illustration of isolation and 

 its effects is unsatisfactorily small. 



An interesting corollary has been recently indi- 

 cated by Professor Cossar Ewart.* Breeding with- 

 in a narrow range often occurs in nature, being neces- 

 sitated by geographical or other barriers. In arti- 

 ficial conditions, this in-breeding often results in 

 the development of what is called prepotency. This 

 means that certain forms have an unusual power of 

 transmitting their peculiarities, even when mated 

 with dissimilar forms. In other words, certain varia- 

 tions have a strong power of persistence. Therefore, 

 wherever through in-breeding (which implies isola- 

 tion) prepotency has developed, there is no difficulty 

 in understanding how even a small idiosyncrasy may 

 come to stay, even although the bridegroom does not 

 meet a bride endowed with a peculiarity like his own. 



In Conclusion. In conclusion, or we should 

 rather say in ending this review whose point is its 

 inconclusiveness, let us once more emphasise that 

 while the general idea of evolution stands more firmly 

 than ever as a reasonable modal interpretation of 

 nature, there is great uncertainty in regard to the 

 factors in the evolution process. How do variations 



* Penycuik Experiments, 1899. 



