THE ORIGIN OF VARIATION 31 



summaries. The most judicious and well informed, though 

 by now a little behind the time, is that of Dctlefsen (1925), 

 which is admirable in its judgment and analysis. It omits 

 some important experimental work (viz. that of Agar, Sumner 

 and Woltereck) and does not discuss some of the circumstantial 

 evidence (e.g. that based on geographical distribution) in 

 detail. The analysis given by Robson (1928), which is largely 

 based on Detlefsen's summary, contains a more detailed 

 reference to these subjects, though the question of ' Dauer- 

 modifikationen ' (p. 35) is only lightly touched on, and it 

 does not include mention of Woltereck's work. The following 

 discussion is largely based on the two studies just alluded 

 to, with an extended consideration of certain circumstantial 

 evidence in addition. 



There is no need for a long account of the historical con- 

 troversy as to the origin of variation. It is enough to say that 

 in the period up to and including the first acceptance of the 

 theory of Natural Selection the heritable effects of environ- 

 mental change or of use-inheritance were freely held, and 

 Darwin himself, as is well known, accepted the idea. 



The theoretical delimitation of the germ-cells from somatic 

 tissues and the idea of the organic integrity of the former were 

 due to Weismann, though he made a concession in favour of 

 * parallel induction ' as the result of his acceptance of Fischer's 

 experiments. Thus the matter stayed (with a few exceptions, 

 mostly among the palaeontologists) until the past two decades, 

 when the matter has again been called into question by the 

 work of Kammerer, Harrison, Przbram, Woltereck and Rensch, 

 and by the advocacy of MacBride in this country. 



Opponents of the theory of the ' inheritance of acquired 

 characters ' and even those who were prepared to accept the 

 possibility that induced variation might be heritable have 

 always found a serious objection in the difficulty of explaining 

 how a modification of the parental soma might be transferred 

 to the germ cells. The experiments of Castle and Phillips 

 (191 1 ) on ovarian transplantation in guinea-pigs have been 

 held to show that germ cells having a given hereditary con- 

 stitution are not modified by being transplanted to a new 

 ' somatic ' environment. These conclusions have been criti- 

 cised by Detlefsen (I.e. p. 257). The latter goes on to show that 

 there is much evidence to prove that our present cytological 



