178 THE entomologist's record. 



or had not appeared at the right places '? Or suppose that thoy had 

 really appeared, hut occurred only in a small percentage of individuals? " 



Such are the questions he suggests as ohjections likely to be raised 

 by the opponents of selection. He says that "it is a fact that in con- 

 stant species, i.e., such as are not in process of transformation, the 

 variations of the markings are by no means frequent or abundant." 

 But is this true ? The minor variations in every species are innum- 

 erable, and in others even striking variations are abundant. It is, of 

 course, necessary to assume, if we are to accept the explanation of 

 mimicry as put forward by Darwin and Wallace, that the useful varia- 

 tions are always present, but, even in the very simplest cases of 

 selection we must make the same assumption, and also assume that such 

 variations always exist in a sufficiently large number of individuals 

 for the selective process. It must also be borne in mind that every 

 primary modification brings in its train secondary ones, which usually 

 occur in harmony with the primary one. 



Having thus reached, as it were, the basis on which mimicry can 

 alone be explained by selective processes, viz., the unfailing presence 

 of useful variations in a sufficient number of individuals for the 

 purposes of selection, Weismann sets himself to discover why it 

 happens that the useful variations are always present, and concludes 

 that " the conception of the selective processes which has hitherto 

 obtained is insufficient, that the root of the process in fact lies deeper, 

 that it is to be found in the place where it is determined what varia- 

 tions of the parts of the organism shall appear — namely, in the germ." 



To enter into the whole process of reasoning as to how it happens 

 that every particle of the germ contains, by its multitude of hereditary 

 transmissions, any particular variation that may be wanted, is outside the 

 scope of this paper. Sufficient to say that every independent part, how- 

 ever small (a single scale for example), is " represented in the germ by a 

 (h'tcn)iinant, that is, by a determinative group of vital units, whoso 

 size and power of assimilation correspond to the size and vigour of the 

 part. These determinants multiply, as do all vital units, by growth 

 and division, and necessarily they increase rapidly in every individual. 

 .... And since there is no more reason for excluding irregularities of 

 passive nutrition, and of the supply of nutriment in these minute 

 microscopically invisible parts, than there is in the larger visible parts 

 of the colls, tissues and organs, consequently the descendants of a 

 determinant can never all be exactly alike in size and capacity of 

 assimilation, but they will oscillate in this respect to and fro about the 

 maternal determinant as about their zero-point, and Avill be partly 

 greater, partly smaller, and partly of the same size as that. In these 

 oscillations, now, the material for further selection is presented, and 

 in the inevitable fluctuations of the nutrient supply I see the reason 

 why every stage attained immediately becomes the zero-point of new 

 fluctuations, and, consequently, why the size of a part can bo aug- 

 mented or diminished by selection without limit, solely by the dis- 

 placement of the zero-point of variation as the result of selection." 



If, therefore, the vital units which make up a determinant are so 

 variable, either of these variable units may be advanced at the expense 

 of the rest, and will be advanced if of use. The variation, therefore, 

 is guided by utility. It matters not whether their primary constituents 

 be called determinants or by some other name, it is of importance to 



