ACQUIRED VARIATIONS 143 



natural, therefore, to suppose that the differences in the colours and 

 markings of the two broods had something to do with the temperature 

 under which their pupae had lived ; and as early as 1864 Dorfmeister 

 proved that the two forms of the tortoiseshell butterfly (as well as 

 an intermediate form occasionally met with) could be obtained by 

 rearing their pupae in different conditions of temperature." 



That is, definite variations were produced as a result 

 of change in environment. But a point of special im- 

 portance is that there is a special period in development 

 when the application of the modifying force produces a 

 maximum effect, as expressed in visible variation. Thus, 

 Merrifield found, after many years of experimenting, that 

 " it is in the larval stage that the life-habits and mass 

 of the two phases are as a rule determined, but in the 

 pupal stage the facies may be affected and even trans- 

 formed." * 



Now, when the modifying force is applied to the growing 

 larva it affects all its cells ; some at particular stages on the 

 road to reproduce the sexual cells from which they originated, 

 and others which are evolving on side-paths to become 

 mature somatic cell-species. Both types of cell are " be- 

 coming " ; in the one case they will become in their final 

 product sexual elements, in the other differentiated tissue 

 cells ; and the action of the modifying force is to influence 

 subtly the process of " becoming," so that when this ends, 

 and all have '' become," there is modification of constitution 

 or identity. As regards the Acquisition of the variation, it 

 is clear that the modifying force must act during develop- 

 ment if results are to be expected, and it is obvious that 

 the earlier its action on the product of the fertilised ovum 

 the more marked will be the results. The force is unable 

 to affect to a recognisable extent the finally evolved fixed 

 tissue cells of the mature insect ; to produce visible effects 

 it must act on the " primitive ancestors " of these cells in 

 the evolving Individual. 



This is not difficult to understand, but the problem of 

 the Transmission of acquired variations is less simple. 



In the paper quoted we find that it was discovered by 

 Professor Fischer that the modification produced by a modified 



1 Inherited Variation in Animals. Prince Kropotkin. " The Nine- 

 teenth Century and After," Nov. 1015. 



