100 ON HEREDITY. [II. 



These changes — such, for example, as are produced by a 

 strange cHmate — have been always looked at under the sup- 

 position that they are transmitted and intensified from genera- 

 tion to generation, and for this reason the observations are not 

 alwaj'S sufficiently precise. It is difficult to say whether the 

 changed climate may not have first changed the germ, and if this 

 were the case the accumulation of effects through the action of 

 heredity would present no difficulty. For instance, it is well 

 known that increased nourishment not only causes a plant to 

 grow more luxuriantly, but it alters the plant in some distinct 

 wa}', and it would be wonderful indeed if the seeds were not 

 also larger and better furnished with nutritive material. If the 

 increased nourishment be repeated in the next generation, a 

 still further increase in the size of the seed, in the luxuriance of 

 the plant, and in all other changes which ensue, is at any rate 

 conceivable if it is not a necessity. But this would not be an 

 instance of the transmission of acquired characters, but only 

 the consequence of a direct influence upon the germ-cells, and 

 of better nourishment during growth. 



A similar interpretation explains the converse change. When 

 horses of normal size are introduced into the Falkland Islands, 

 the next generation is smaller in consequence of poor nourish- 

 ment and the damp climate, and after a few generations they 

 have deteriorated to a marked extent. In such a case we have 

 only to assume that the climate which is unfavourable and the 

 nutriment which is insufficient for horses, affect not only the 

 animal as a whole, but also its germ-cells. This would result 

 in the diminution in size of the germ-cells, the effects upon 

 the offspring being still further intensified by the insufficient 

 nourishment supplied during growth. But such results would 

 not depend upon the transmission by the germ-cells of certain 

 peculiarities due to the unfavourable climate, which only appear 

 in the full-grown horse. 



It must be admitted that there are cases, such as the climatic 

 varieties of certain butterflies, which raise some difficulties 

 against this explanation. I m^'self, some years ago, experi- 

 mentally investigated one such case ^ and even now I cannot 



' ' Studien zur Descendenztheorie, I. Ueber den Saison-Dimorphismus 

 der Schmetterlinge.' Leipzig. 1875. English edition translated and 

 edited by Professor Meldola, ' Studies in the Theory of Descent,' Part I.. 



