xix 



an advantageous variation, which gives rise to the selection of a new 

 race or species, may be accompanied by others which are indifferent, 

 but which are just as strongly hereditary as the advantageous varia- 

 tions. The advantageous structure is but one product of a modified 

 general constitution which may manifest itself by several other 

 products; and the selective process carries the general constitution 

 along with the advantageous special peculiarity. A given species of 

 plant may owe its existence to the selective adaptation of its flowers 

 to insect fertilisers ; but the character of its leaves may be the result 

 of variations of an indifferent character. It is the origin of varia- 

 tions of this kind to which Darwin refers id his frequent reference to 

 what he calls " laws of correlation of growth " or " correlated 

 variation." 



These considerations lead us further to see the inappropriateness of 

 the objections raised to Darwin's theory on the ground that natural selec- 

 tion does not account for the first commencements of useful organs. 

 But it does not pretend to do so. The source of such commencements 

 is necessarily to be sought in indifferent variations, which remain 

 unaffected by selection until they have taken such a form as to 

 become utilisable in the struggle for existence. 



It is not essential to Darwin's theory that anything more should be 

 assumed than the facts of heredity, variation, and unlimited multipli- 

 cation ; and the validity of the deductive reasoning as to the effect of 

 the last (that is, of the struggle for existence which it involves) 

 upon the varieties resulting from the operation of the former. Nor 

 is it essential that one should take up any particular position in 

 regard to the mode of variation, whether, for example, it takes 

 place per saltum or gradually ; whether it is definite in character 

 or indefinite. Still less are those who accept the theory bound 

 to any particular views as to the causes of heredity or of variation. 



That Darwin held strong opinions on some or all of these points 

 may be quite true ; but, so far as the theory is concerned, they must be 

 regarded as obiter dicta. With respect to the causes of variation, 

 Darwin's opinions are, from first to last, put forward altogether tenta- 

 tively. In the first edition of the 1 Origin,' he attributes the 

 strongest influence to changes in the conditions of life of parental 

 organisms, which he appears to think act on the germ through 

 the intermediation of the sexual organs. He points out, over and 

 over again, that habit, use, disuse, and the direct influence of con- 

 ditions have some effect, but he does not think it great, and he draws 

 attention to the difficulty of distinguishing between effects of these 

 agencies and those of selection. There is, however, one class of 

 variations which he withdraws from the direct influence of selection, 

 namely, the variations in the fertility of the sexual union of more or , 

 less closely allied forms. He regards less. fertility, or more or less 



