ORGANIC DIFFERENTIATION 93 



development tachygony. In the same series of organisms it is 

 possible to find all the steps intermediate between tachygony 

 and patrogony. This gradual embryogenetic acceleration 

 we have already termed tachy genesis. 1 



Alike in the case of patrogony and tachygony the embryo 

 is subjected to the influence of the actual conditions under 

 which its development takes place. These tend to make it 

 vary from the ancestral forms, and may impose upon it forms 

 quite different from those of the embryos of species that have 

 developed under other conditions. These adaptive modes of 

 development, which may be recognized by the great variety 

 of characters that the embryos of related species present, are 

 distinguished under the name armogony, 2 and the phenomenon 

 of adaptation to the conditions of development that determine 

 them can thus be called armogenesis. Armogenesis can both 

 complicate and simplify patrogony. Thus, pelagic embryos 

 living in the open sea often acquire organs which are not 

 present in the littoral patrogonic embryos ; on the other hand, 

 the larvae of insects which live parasitically lose the legs and 

 even the masticatory organs found in free larvae. Moreover, 

 the very simplified tachygonic embryos of mammals develop 

 a special organ, the placenta, which has nothing to do with 

 ancestral forms. 3 



It might perhaps be surprising that armogenesis, while 

 modifying embryonic forms, should not lead to important 

 modifications in the final form they are to assume. Giard has 

 endeavoured to explain this paradox by resorting to 

 comparison with mechanical laws. Comparisons, however, are 

 not reasons. In reality it is the elements held in reserve in the 

 interior of the body, the highest expression of which is found in 

 the histoblasts of insects and Nemerteans, which mould 

 the permanent form after the disappearance of armogonic 

 organs, because these reserves have escaped from the 

 influence of external actions and have preserved intact the 



1 LXXVIII, 149. 



1 From ap/xos joint or app-i] union and consequently adaptation and 

 yovos, generation. I have hitherto employed the word armozogony from the 

 verb app.6£a> " I harmonize," but it does not express the facts any better, 

 and is too long. 



3 It should, of course, be remembered that the words patrogenesis, 

 tachygenesis, and armogenesis by no means designate a special active 

 cause found only in living beings, but the totality of the causes and 

 mechanisms, still too little known, which give rise to the diverse modes of 

 organic development. 



