PRESIDENTIAL ADDRESS SECTION D. 175 



cases put forward in the course of the last year or so, may be 

 mentioned the explanation by Stahl of the existence, in the 

 cells of many plants, of very numerous bundles of raphides 

 or needlelike crystals by the experimental proof that the 

 plants in question are thereby protected against the attack of 

 snails ; the explanation of the remarkable colour and markings 

 of certain male spiders by the observation that these decora- 

 tions are prominently displayed during courtship ; and the 

 demonstration by Prouho of the defensive function of those 

 problematical organs the pedicella-rite of sea-urchins. So many 

 things have been explained by this principle of utility to 

 the individual or to the race that there can be no doubt that 

 as we know more of the details we shall be able to explain 

 very much more in the same way. 



It is the explanation which it affords of the causal connec- 

 tion between the utility of a part and its development that 

 constitutes the main feature of Darwin's theory, as distin- 

 guished from such theories as von Naegeli's, which presuppose 

 an internal transforming force. In the struggle for existence, 

 according to Darwin, useful variations in parts are preserved 

 owing to the advantage which the favoured individuals possess 

 over others of the same race in virtue of the variations in question, 

 such useful variations being then transmitted by inheritance, 

 and by-and-by becoming more and more marked by successive 

 repetitions of the same process. It is known as a fact of com- 

 mon observation that variations are constantly arising ; and 

 the facts which Darwin brought together regarding the re- 

 sults of the artificial selection of such variations in the case of 

 domestic animals showed how considerable the effects of such 

 a process might be if carried on for. a sufficient length of time. 

 And hence Darwin's conclusion that we have in this phenome- 

 non of va^riation, together with the supposition that Nature (by 

 which is meant the conditions of existence of the organism) 

 selects advantageous variations, the explanation of the pro- 

 gress and improvement of the organism, and eventually of the 

 whole animal and vegetable world. 



But how do the variations arise ? That they do present 

 themselves is a matter of every-day observation ; but the 

 causes to which their appearance is due are very obscure. 

 Darwin himself, in fact, attempted no detailed explanation of 

 this. It is a fact that organisms tend to vary — their insta- 

 bility being in many cases very great — and for the theory of 

 natural selection this is enough. A further question in con- 

 nection with the development of organisms is the question 

 how far what affects the organism during life — the action of 

 the environment, and the effects of the activities of the organ- 

 ism on the development of special parts — may have a share in 

 the process of evolution. As is well know'u, Lamarck attri- 



