492 THE POPULAR SCIENCE MONTHLY. 



tion tends to form varieties by peculiarities in the reproductive 

 system of individuals, which render them unfit for perfect union, 

 or cause them to remain more or less sterile, with other individ- 

 uals which have not the same peculiarities. 



The exact reasons are recondite, and the whole subject difficult 

 of demonstration except from the results, since changes in the re- 

 productive organs are not easily observable. Romanes believes 

 this sterility to be incidental to variation, and hence one of the 

 chief causes of the accumulation of such variation. Wherever 

 there has been modification of the reproductive organs intro- 

 ducing incompatibility between two individuals, even where there 

 has been no other change or variation, we have a valid cause of 

 differentiation which in its consequences must be important. Com- 

 patibility or fertility between individuals is of the very essence of 

 selection. Natural selection implies that this sexual divergence is 

 subsequent to or coincident with divergences in other directions ; 

 physiological selection, that it antecedes them. To put the case 

 of Romanes more fully, we will suppose that among the natural 

 variations there occasionally occurs something to affect the repro- 

 ductive organs in such wise as to produce incompatibility — i. e., 

 incapacity of one individual with another of the parent type to 

 unite, or sterility of such union, while it remains fertile with the 

 variation of its own kind. This theory, of course, implies variation 

 in the reproductive organs, or departure from the parental type, in 

 at least two individuals of opposite sex simultaneously, and with 

 this admission, for which we are justified in facts, physiological 

 selection will preserve many j)eculiarities which need have no 

 necessary connection with the exigencies of life. 



The change may be in the organs of reproduction, introducing 

 sexual incompatibility, or it may be due to other causes, as, for 

 instance, the time of flowering in plants, or the season of heat in 

 animals. Even the element of scent becomes important here, as 

 my friend J. Jenner Weir has suggested, since it may influence 

 sexual relationship, so that the very excretions of the body, which 

 vary with individuals, must be allowed their part. Francis Galton 

 has indicated a modification of Romanes's views, viz., that the pri- 

 mary characteristic of a variety resides in the fact that the indi- 

 viduals who compose it do not care to mate with those outside 

 their pale. Incipient varieties are thus thrown off from the parent 

 stock by means of peculiarities of sexual instinct which prompt 

 what anthropologists call endogamy, and check exogamy or mar- 

 riage without the tribe or caste. This is a very good anthropologi- 

 cal illustration of how physiological selection may begin. 



Natural selection preserves the individuals best adapted to 

 life-conditions by destroying the less fit. Physiological selection 

 may be said to preserve differences which have no necessary con- 



