1899] THE SCOPE OF NATURAL SELECTION 117 



for by the single principle of survival of the fittest and elimination of 

 the less fit, they were justified in considering natural selection to be 

 the main or sole principle in species formation. In the other Neo- 

 Lamarckians based their objections to natural selection on the assump- 

 tion that modifications in nature were always or nearly always definite, 

 that definite modifications were admittedly unexplainable on the 

 selectionist theory, it therefore followed, as nature could produce 

 definite modifiability, without the aid of natural selection, that, unless 

 some special and additional reason could be found for its existence, 

 the selectionist principle must be regarded as wholly subsidiary in 

 nature, and that it could only be regarded as a species-former in the 

 limited field of the domesticated organisms which were under the 

 direct influence of man. Neither position could be regarded as 

 satisfactory, siDce each school of thought was apparently supported by 

 some facts, while negatived by others. Professor Lloyd Morgan, in an 

 article contributed to Natural Science in 1892, altered the whole 

 force of the arguments advanced on both sides by demonstrating the 

 fact that if natural selection acts at all, it must tend, under moderately 

 constant conditions, to produce definite variability through survival of 

 the favourable line of inheritance, and extermination of the unfavour- 

 able. This corollary to the principle of selection he has further 

 expounded in his work on " Habit and Instinct " in a chapter entitled 

 " Modification and Variation." 



In an article published in this journal for April 18981 contended 

 that natural selection was capable of producing in the whole organism 

 a general definite variability under relatively constant conditions. I 

 was at that time unaware that Professors Lloyd Morgan and 

 Weismann l had both in large part anticipated me. 



The former writer's views may be summarised briefly as follows : — 



The theory of natural selection, involving as its fundamental prin- 

 ciple the assumption that an organism survives solely because it has 

 certain favourable elements in its nature which give it certain advan- 

 tages in the competition for existence, the less favoured organisms 

 being eliminated, it follows, in so far as parental characteristics are 

 able to influence those of their offspring, that the progeny of successful 

 parents will be likely to inherit a higher average of adaptability to 

 their environment, and as this average adaptability will keep rising 

 so long as selection lasts, it will tend, under more or less constant 

 conditions, to produce more or less definite variability. Definite 

 variability is not therefore necessarily inconsistent with the principle 

 of selection. If it exists only where the conditions are such that the 

 principles of the theory would lead any impartial biologist to expect 

 such definite variability it will be strong confirmation of the truth of 

 the theory in question. 



Every living organism may be considered from two aspects — (1) it 



1 In his theory of "Germinal Selection " put forward in September 1895 at Leyden. 



