20 



THE DIFFEEENTIATION OP SPECIES. 



The law of 

 " like pro- 

 duces like." 



Swamping 

 effects of 

 inter- 

 breedinff. 



Physiologi- 

 cal Selection 

 defined by 

 liomanes. 



Definition of 

 the Author. 



is not accidental. Variation is, to a large extent, the result of the difference in the forces 

 which are brought to bear upon the individual, or upon the organ, between conception and 

 puberty. Like produces like, and under like circumstances offspring would doubtless resemble 

 their parents. But the circumstances never are alike. The state of health of the parents, the 

 quality and quantity of the food, the variations of chraate, the amount of use or disuse of each 

 organ that circumstances may demand : these and a hundred other influences determine the 

 points in which the offspring shall exceed or fall short of, in fact shall vary from, their parents. 

 The result is that every generation starts from a slightly different platform to the last. It is 

 this variation which to a large extent produces Evolution, or descent with modification. 

 The variation is perpetuated because it occurs in many ; perhaps, one might say, more or 

 less in all the individuals in one district at the same time, for hke causes produce like 

 effects. There can be no doubt that the effects of this variation are hastened by Natural 

 Selection ; the unfit are continually being weeded out in the struggle for existence ; but 

 descent with modification can proceed without any struggle for existence, though the 

 modification is not so rapid. But neither variation nor natural selection can originate a 

 new species alone. The ancestral ape might have become modified into man in the course of 

 ages without a second species having been produced. To understand the differentiation of 

 species, we must discuss Mr. Romanes's other difficulty. This " consists in the swamping 

 influence upon an incipient variety of free intercrossing." This is unquestionably a very 

 grave difficulty, to my mind an absolutely fatal one, to the theory of accidental variation. 

 The force of the difficulty is admirably illustrated by Mr. Romanes by a quotation from the 

 ' North British Review,' in which a white man is supposed to be stranded on an island 

 inhabited by negroes, with whom he would intermarry, and where the result is fairly 

 assumed to be, not the gradual whitening of the negroes on that island, nor the origination 

 of a white species or variety, but the gradual swamping of the white strain by intermarrying, 

 until, after a sufficient number of generations, it disappeared altogether. 



Mr. Romanes proposes to modify the theory of Natural Selection so as to remove 

 these three difficulties, by adding to it a theory, to which he gives the name of " Physiological 

 Selection." The new theory is described as follows : — " Wherever, among all the possible 

 variations of the highly variable reproductive system, there arises towards any parent form 

 any degree of sterility which does not extend to the varietal form, there a new species must 

 necessarily take its origin." This proposition is true, if for the word must be substituted 

 way, under certain circumstances. To make the theory of " Physiological Selection " tenable 

 it must be stated as follows : — 



Amongst the innumerable accidental variations which are constantly being produced 

 by unknown causes, and which are invariably swamped by interbreeding, there sometimes 

 arises a variation in the reproductive organs, preventing the possessor of it from beino- 

 fertile with the rest of the species. Of the innumerable variations of which the reproductive 

 organs are capable, a similar, or rather a corresponding, variation sometimes occurs 

 simultaneously in another individual of the opposite sex, and sometimes these two individuals 



