PBESIDENTIAL ADDRESS — SECTION D. 183 



siou that the markings go through a quite regular series of 

 changes during the hfe-history of the individual — longitudinal 

 stripes, black spots, black rings, ocelliform markings. The 

 varieties of markings which we find in the adults of the same 

 species are produced by one individual or variety stopping 

 short, as regards its changes of pattern, at one stage, another 

 at another, so that we have striped, dotted, black-ringed, and 

 ocellate varieties. New variations make their appearance 

 mainly in old males, after the ordinary changes have all 

 been gone through, and the males transmit them to their 

 progeny. 



Eimer adduces many interesting facts relating to variations 

 of markings, influence of light on colour, instinct and intelli- 

 gence, and many other topics having a bearing on the question 

 of the evolution of plants and animals. But the support which 

 these give to the wide generalisations above quoted is very 

 small. That external conditions have certain direct effects 

 on organisms is a matter of every-day observation. But these 

 effects, so far as w^e have evidence, are almost all of a super- 

 ficial character ; and the few cases which Eimer mentions of 

 more profound effects apparently resulting from the action of 

 external conditions — that of the axolotl, and that of the brine- 

 shrimp, for example - — are more satisfactorily explained by 

 degeneration and reversion than as instances of evolution 

 per saltum brought about by the direct action of external in- 

 fluences. 



An important and interesting attempt to explain the 

 phenomena of heredity is that which has been made by de 

 Vries, the botanist of Amsterdam, in a work called "Intracel- 

 lular Pangenesis," published in 1889. Darwin's theory of 

 pangenesis, which he himself did not put forward with much 

 confidence, has not found many adherents, though it was emi- 

 nently useful in formulating a problem demanding solution. A 

 modification of this theory by W. K. Brooks avoided some of 

 the difficulties which Darwin's ideas involved, but still has 

 shown itself open to exception on various grounds, as has been 

 well shown among others by Weismann in his essays. Accord- 

 ing to Darwin's theory the cells of the body under ordinary 

 conditions give off niinute particles or gemmules, which find 

 their way to tlic reproductive cells, and there become collected. 

 It is the presence of these minute particles that endows the 

 reproductive cells with the faculty of reproducing the parent 

 organism : each gemmule is capable of giving rise, when the 

 ovum in which it is contained developes, to that part of the 

 organism from which it originated. But, as has been pointed 

 out by various physicists, the number of such gemmules— 

 each several times larger than a chemical molecule — that can be 



