74 J. W. SLATER, ESQ., P.C.S., F.E.S., ON 



must know " how it behaves under its new conditions " when it is 

 there, and this is the postulate required for the complete statement 

 of the problem. In this manner we open up an illimitable field for 

 experiment and observation. A Bidens cernua cultivated for a series 

 of generations in the tropics might tell a rather strange story con- 

 cerning the antecedents of the species already established in that 

 region. 



Mr. Joseph John Murphy writes : — 



I wish to offer a few observations on Mr. Slater's paper on " The 

 weak sides of Natural Selection." 



I agree with his main conclusion, which I understand to be that 

 although natural selection is an agent in the origin of species, it 

 is by no means the sole or the chief agent; but in some ways he 

 seems unjust to the theory of natural selection, by demanding 

 that it should explain what in the nature of things it cannot 

 explain. 



He says : — " Suppose a pair of animals in the primeval world 

 had produced a hundred fertile ova. The young animals springing 

 from these ova must either be one and all exactly alike, or they 

 must exhibit certain differences. In the former alternative there 

 is no ground for natural selection to work upon ; the very idea of 

 selection implying differences in the objects among which a 

 selection is to be made. In the second alter-native, the varieties 

 (he means variations) being, by hypothesis, antecedent to selection, 

 cannot be its effects. Hence, in either case, we have something 

 which the Darwinian theory is quite unable to account for." This 

 is perfectly true, and perfectly irrelevant. It is like objecting to 

 the Newtonian theory of the planetary motions that it does not 

 account for gravitation ; an objection which, I believe, was actually 

 made in Newton's time. Every theory, except in pure logic and 

 mathematics (and I am not sure that geometry ought to be 

 excepted) must postulate facts — and not only particular facts but 

 general truths — without being able to account for them. The next 

 observation, that " before we can understand the origin of species, 

 we want a law which shall go deeper than natural selection," is as 

 true and as luminous as if he had said " we want a law which 

 shall go deeper than gravitation before we can understand the 

 motions of the planets." To such objections it is enough to reply 

 that gravitation is ultimate in astronomy, and spontaneous 

 variation ultimate in morphology and evolution. 



In another passage, Mr. Slater appeal's to have not only 

 mistaken the logic, but the meaning, of the qnestion under dis- 

 cussion. He says Mr. Wallace " admits that the most effective 

 agent in the extinction of species is the pressure of other species, 

 whether as enemies or simply as competitors — a distinction, I must 

 remark, without a difference." No difference between enemies and 

 competitors ! If sheep were exterminated in one country through 



