PEOPLING OF LAND AND SEA 195 



comes in contact, even though they may be separated 

 from it. 



The wonderfully energetic action exercised by minute doses 

 of what are known as internal secretions, is only a particular 

 instance of this general phenomenon. The activity of an organ 

 does not modify that organ alone ; it may react on the whole 

 organism, as in the well-known instance of the maturation of the 

 ovum. It can determine modifications in other organs, and 

 become the cause of unexpected variations in characters, and 

 herein, perhaps, lies the secret of one part at least of those 

 sudden variations pointed out by Charles Naudin in 1865, upon 

 which was based a doctrine subsequently credited to De Vries. 

 It is the collaboration by means of their excretions of all these 

 structural elements in the formation of the internal environ- 

 ment, partly with the aid of the nervous system, and partly 

 in independence of it, which establishes the solidarity 

 characteristic of higher individualities. Every modification in 

 the chemical constituents of one of these elements may have 

 its echo in others, and, as Armand Gautier has shown, can 

 even modify the forms of living beings ; so much so that, 

 at some future date, morphology may be entirely bound up 

 with the chemistry, still so mysterious in many respects, of the 

 albuminoid compounds, diastases, and numerous other sub- 

 stances, to which, for lack of better knowledge, we now give 

 the vague names toxins, hormones, etc. 



On the other hand, every time that two organisms 

 enter into permanent relations with each other they 

 gradually modify each other by reason of these very 

 relations ; this is what I described in 1881 x as the reciprocal 

 adaptation of organisms. A parasite is modified by its sojourn 

 inside its host ; but to an equal extent it modifies the body in 

 which it lives. Allmann in 1871 2 had already called attention 

 to the fact that the larvae of the Pycnogonida, which lodge as 

 parasites in Hydroids, give to the host merids that 

 nourish them the aspect of reproductory merids, and I 

 myself wrote in regard to this subject : " When the growing 

 reproductive organ attracts to itself the nutritive fluids, is it 

 not acting in the same way as a parasite which turns to its 

 own profit a part of the digestive activity of the polyp ? '\ 3 

 In fact the presence of the parasite often excludes all develop- 



1 XXVII, 710. 2 LXXIII, 40. 3 XXVII, 234. 



