272 THE EVOLUTION THEORY 



modifications, any transmission of the acquirements of the personal 

 part to the germinal part, were possible at all, it would occur in this 

 case, for many species of gall-insects attack plants, particularly oaks, 

 in great numbers every year. It has actually been maintained 

 that galls may arise spontaneously, that is without the presence 

 of a gall-insect. But no proof of this has ever been found, and the 

 fact that no one has paid any attention to the assertion probably 

 implies an unconscious condemnation of the hypothesis of the trans- 

 7 missibility of acquired characters. 



It has been proved by Nageli's often discussed experiments on 

 hawkweeds (Hieraclum) th&t much less specialized external influences 

 can give rise to changes which are not hereditary. The Alpine 

 species of hawkweed varied considerably in their whole habit in the 

 rich soil of the Botanic Gardens at Munich, Ijut their descendants, 

 when transferred to a poor flinty soil, returned to the habit of the 

 Alpine species. The changes which occurred in garden soil were 

 therefore somatic and, as I have called them, ' transient,' and they 

 did not depend upon variations of the germ-plasm. It may be 

 objected in regard to these experiments that they were not continued 

 long enough to prove that hereditary variations would not also have 

 cropped up in consequence of the altered conditions. But in any 

 case they prove that marked changes in the whole bod}" of the plant 

 may occur without any obvious variation of the germ-plasm. This 

 does not mean, however, that the possibility of \'ariations of the germ- 

 plasm through such direct external influences is disputed. We must 

 assume the occurrence of these on a priori grounds, if we refer — as 

 we have done — individual hereditary variation to fluctuations in the 

 nutrition of the individual determinants of the germ-plasm. It is 

 probable that many general nutritive variations or climatic factors 

 affect the germ-plasm as well as the soma, and it is b}^ no means 

 inconceivable that it is not all, but only certain definite determinants 

 that are caused to vary. 



A proof of this may be found in the results of experiments made 

 upon the little red-gold fire-butterfly (Polyommatus jMceas), to which 

 I have briefly referred in a former lecture. This little diurnal 

 buttei'fly of the family Lycasnida; has a wide distribution and occurs 

 in two climatic varieties. In the far north and also in the whole 

 of German}^ the upper surface is red-gold with a narrow black outer 

 margin, but in the south of Europe the red-gold has been almost 

 crowded out by the black. I reared caterpillars in Germany from 

 eggs of P.'pJdoicts found at Naples and exposed them directly after 

 they had entered on the pupa-stage to a relatively low temperature 



