428 



DARWINISM 



CHAP. 



elements, and in this way may iDrofoundly modify the whole 

 organisation. Why and how the external effects are limited 

 to special details of the structure we do not know ; but it does 

 not seem as if any far-reaching conclusions as to the cumula- 

 tive effect of external conditions on the higher terrestrial 

 animals and plants, can be drawn from such an exceptional 

 phenomenon. It seems rather analogous to those effects of 

 external influences on the very lowest organisms in which the 

 vegetative and reproductive organs are hardly differentiated, 

 in which case such effects are doubtless inherited. ^ 



Professor Geddes's Theory of Variation in Plants. 



In a paper read before the Edinburgh Botanical Society in 

 1886 Mr. Patrick Geddes laid down the outlines of a funda- 

 mental theory of plant variation, which he has further ex- 

 tended in the article "Variation and Selection" in the 

 Encijdopmdia Britannica, and in a paper read before the Linnsean 

 Society but not yet published. 



A theory of variation should deal alike with the origin of 

 specific distinctions and with those vaster differences which 

 characterise the larger groups, and he thinks it should answer 

 such questions as — How an axis comes to be arrested to form 

 a flower 1 how the various forms of inflorescence were evolved ? 

 how did perigynous or epigynous flowers arise from hypogynous 

 flowers ? and many others equally fundamental. Natural selec- 

 tion acting upon numerous accidental variations will not, he 

 urges, account for such general facts as these, which must 

 depend on some constant law of variation. This law he 

 believes to be the well-known antagonism of vegetative and 

 reproductive growth acting throughout the whole course of 

 plant development ; and he uses it to explain many of the 

 most characteristic features of the structure of flowers and 

 fruits. 



^ In Dr. Weismann's essay on " Heredity," already referred to, he considers 

 it not improbable that changes in organisms produced by climatic influences 

 may be inherited, because, as these changes do not affect the external parts 

 of an organism only, but often, as in the case of warmth or moisture per- 

 meate the whole structure, they may possibly modify the germ -plasm 

 itself, and thus induce variations in the next generation. In this way, he 

 thinks, may possibly be explained the climatic varieties of certain Initterflies, 

 and some other changes ^vhich seem to be effected by change of climate in a 

 few generations. 



