356 ME. G. J. ROMANES ON PHTSIOLOGHCAL SELECTION. 



selection is not necessarily concerned with tlie causes of variation 

 in the reproductive system, it will be convenient to classify these 

 causes as extrinsic and intrinsic. By the extrinsic causes I mean 

 changes in the environment which act upon the reproductive 

 system, whether these be changes of food, climate, degree of 

 liberty, and so forth. By intrinsic causes I mean changes taking 

 place in the reproductive system itself of a kind depending on 

 what Mr. Darwin calls " the nature of the organism," or on causes 

 which we are not able to trace, and which may therefore be termed 

 spontaneous. 



Now the particular kind of variation the occurrence of which 

 I have to prove is that of impotency — whether absolute or com- 

 parative — towards the parent form, without decrease of potency 

 towards the varietal form. One very obvious example of this kind 

 of variation has already been given in the season of flowering or 

 of pairing being either advanced or retarded. This I conceive to 

 be a most important case for us, inasmuch as it is one that must 

 frequently arise in nature. Depending, as it chiefly does, on ex- 

 ternal causes, numberless species both of plants and animals must, 

 I believe, have been segregated by its influence. For in every 

 case where a change of food, temperature, humidity, altitude, oi- 

 of any of the other many and complex conditions which go to 

 constitute environment — whether the change be due to migration 

 of the species, or to alterations going on in an area occupied by a 

 stationary species — in every case where such a change either pro- 

 motes or retards the season of propagation, there we have the 

 kind of variation which is required for physiological selection. 

 And it is needless to give detailed instances of its occurrence 

 where this is due to so well-known and frequently-observed a 

 cause. 



But it is in what I have called the spontaneous variability of 

 the reproductive system itself that I mainly rely for evidence of 

 physiological selection. The causes of variability are here far 

 more numerous, subtle, and complex than are such extrinsic 

 causes as those above mentioned ; and they are always at work in 

 the reproductive systems of all organisms. Moreover, sensitive 

 as the reproductive system is to small changes in the conditions 

 of life, its spontaneous variability is, as Mr. Darwin has shown, 

 even more remarkable. Now, among all the possible variations 

 of the reproductive system however caused, there is one which, 

 whenever it is produced, cannot be allowed again to disappear ; 



