BASIS OP THE PEOBLEM 73 



death-rate. When, as is usually the case, biparental reproduction 

 takes place, the position is more complicated : strains are being 

 continually crossed, and, as the result of the chance mixture of 

 factors in the germinal constitution of the children, the offspring 

 of the same parents differ in their germinal constitution one 

 from the other. Therefore, in any species in which biparental 

 reproduction takes place, new combinations are constantly 

 arising ; but, though in this manner the germinal constitu- 

 tion may in a sense change, the change is due simply to 

 a shuffling of factors. What we require now to ask is what is 

 known as regards the manner in which new factors arise and are 

 added to the germinal constitution, and the manner in which old 

 factors drop out and are lost from the germinal constitution ; 

 for it is only owing to such additions and to such losses that true 

 germinal change occurs, and that shuffling is rendered possible. 



6. The interpretation of the results of the crossing of different 

 strains has shown what kind of changes underlie the appearance 

 of certain mutations. It has been shown that many of the varieties 

 of domesticated species have originated by the apparent loss of 

 one or more unit-factors.^ Thus the numerous varieties of domestic 

 rabbits and of sweet-peas are all descended from a single wild 

 species of rabbit and of pea, and differ from the wild stock not by 

 an addition to, but by an apparent subtraction from, the total 

 number of factors in the germinal composition of the wild stock. 

 The reasoning which has led to this conclusion need not be 

 followed here ; one proof is that when certain varieties are 

 crossed, characters of the original stock reappear, due to the fact 

 that, one variety having apparently lost one factor and the other 

 another, crossing results in the recombination of the factors 

 necessary to the manifestation of the original character. 



This is a strange conclusion, but it seems nevertheless to be 

 true that in this manner many domestic varieties have arisen. 

 If this was the only manner in which mutations could come about, 

 then we should be driven to imagine that the most elementary 

 form of life contained within it innumerable factors, and that 

 evolution has merely consisted in the apparent dropping out of 

 factors. This conclusion has indeed been tentatively suggested. 

 But until it has been definitely shown that this is the only manner 



' There is probably no actual loss — no gap in a chromosome. It may be 

 supposed that owing to a ' negative variation ' a factor ceases to be functional. 



