TRANSACTIONS OF SECTION D. 667 



had little influence, it fell to Charles Danviii to ooustriict new and more lasting 

 foundations for the evolution theory. 



Charles Darwin, clearly realising tliat variation occurs in many different 

 directions, arrived at the far-reaching conclusion that the- best adapted varieties 

 are selected by the environment, and thus have a chance of giving- rise to new 

 species. Tliough impressed with the paramount importance of selection, Charles 

 Darwin realised that ' its action absolutely depends on what we in our ignorance 

 call spontaneous or accidental variation." ' Darwin, however, concerned himself 

 to the last more with selection than with variation, doubtless because he believed 

 variability sinks to a quite subordinate position when compared with natural 

 selection. As variations stand in very much the same relation to selection as 

 bricks and other formed material stand to the builder, Darwin was perhaps 

 justified in rating so highly the importance of the principle with which his name 

 will ever be intimately associated. Though Darwin considered variability of 

 secondary importance, it may be noted that he did more than any other naturalist 

 to collect the facts of variation, and he, moreover, considered at some length the 

 causes of variation. He regarded with most favour the view ' that variations of 

 all kinds and degrees are directly or indirectly caused by the conditions of life 

 to which each being or more especial^ its ancestors have been exposed.' - Of all 

 the causes which induce variability, he believed excess of food was probably the 

 most powerful.^ In addition to variations which arise spontaneously in obedience 

 to fixed and immutable laws Darwin believed with Bufibn that variations were 

 produced by the direct action of the environment, and with Lamarck by the use 

 and disuse of parts ; and he accounted for the inheritance of such variations by bis 

 theory of pangenesis. Darwin seems always to have regarded the direct action of 

 the environment and use and disuse as, at the most, subsidiary causes of variation ; 

 but Mr. Herbert Spencer and his followers regard ' use-inheritance ' as an all- 

 important factor in evolution ; while Cope and his followers in America, by a 

 mixture of ' use-inheritance '(Kinetogeneis) and Lamarck's neck-stretching theory 

 (ArchDesthetism), apparently see their way to account for the evolution of animals 

 with but little help from natural selection. 



Professor Weismann and others, however, have recently given strong reasons 

 for the belief that all variation is the result of changes in the germ-plasm ultimately 

 due to external stimuli, the environment acting directly on unicellular, indirectly 

 on multicellular organism. It is convenient to speak of biologists who believe 

 with Mr. Herbert Spencer in the law of use and disuse (use-inheritance) as Neo- 

 jjamarckians, and of those who with Weismann refuse to accept the doctrine of 

 the transmission of definite acquired characters, and in the case of multicellular 

 organisms the direct influence of the environment as a cause of variation, as Neo- 

 Darwinians. In discussing variability I sluill assume that all variations are 

 transmitted by the germ-cells ; that the primary cause of variation is always the 

 effect of external influences, such as food, temperature, moisture, &c. ; and that 

 'the origin of a variation is eqiially independent of selection and amphimixis,' ' 

 amphimixis being simply the means by which effect is given to differences 

 inherited, and to the differences acquired by the germ-cells during their growth 

 and maturation. 



Theoretically the offspring should be an equal blend of the parents and 

 (because of the tendency to reversion) of their respective ancestors. In as far 

 as the offspring depart either in an old or in a new direction from this ideal 

 intermediate condition they may be said to have undergoue variation. The 

 more obvious variations consist of a difference in form, size, and colour, in the rate 

 of growth, in the period at which maturity is reached, in the fertility, in the power 

 withstand disease and changes in the surroundings, of differences in temperament 



' Aniiiiali and Plants, vol. ii. p. 20G. 



- Hid., vol. ii. p. 240. Elsewhere he says we are ' driven to the conclusion 

 that in most cases the conditions of lite play a subordinate part in causing any 

 particular modilication.' 



' Ihid., vol. ii. p. 283. 



* Wpismann, The Germ-PUntin. \). 4.31, 



