the Convergence of Types by Pelagic Life. 83 



bring into the light this power of ethology. It will be the 

 glory of Darwin that he has shown how much this action of 

 the surrounding media is increased by natural selection, the 

 idea of which is essentially inseparable from that of adaptation, 

 selection at a given moment being determined by the limits of 

 this adaptation. 



We shall see hereafter how, in the particular instance of the 

 Chsetognatha and in some other interesting cases, pure adap- 

 tive analogies have been taken for relations of affinity. With- 

 out pretending to give a complete solution of these questions, 

 which are too complex to be treated lightly, we shall esteem 

 ourselves fortunate if we have indicated the nature of certain 

 difficulties, and thus contributed to clear the road which our 

 successors will have to traverse. 



Convergence of Types by Pelagic Life. 



In a previous memoir* I have dwelt upon the convergence 

 of types by parasitism, and pointed out that this mode of ex- 

 istence gradually brought about in the most diverse animals 

 organic modifications so profound as to cause the disappearance 

 not only of the characters of orders and classes, but even of 

 those of the great divisions or subkingdoms. Without the 

 clue furnished by embryogeny we might easily be led to create 

 families and genera including animals belonging to groups so 

 distinct as the Trematoda, the Nudibranchiate Mollusca, the 

 Cirripedes, and the Isopod and Copepod Crustacea. Since 

 then, during the Scientific Congress at Lille, I have had the 

 extreme satisfaction of learning that these opinions were shared 

 by one of the most distinguished embryogenists of our time, 

 Professor Carl Vogt. This eminent philosophical zoologist, 

 without any knowledge of the memoir to which I have alluded, 

 enunciated the same proposition, supporting it by precisely the 

 same examples (Sacculina, Entoconcha, Pedim). 



Opinions of the same nature have also been expressed by 

 Professor Martins (of Montpellier) , one of the few French 

 naturalists who have been able to understand the modern spe- 

 cific movement in the biological sciences. He saysf: — "I 

 cannot refrain from observing that the appearance of the same 

 morphological type (of the same animal, so to speak) at various 

 grades in the scale, is another argument in favour of commu- 

 nity of origin combined with subsequent modifications. The 



* Revue Scieutifique, July 11, 1874, 4 e anne"e, 2 e serie, no. 2, pp. 32 & 

 33. 



t See C. Martins, 'La Creation du monde organise d'aprea les natura- 

 listes de la nouvelle Ecole,' p. 15. 



6* 



