Parasitism and symbiosis — caulleky. 407 



stages were known. The digestive tube of these adult Monstrillidae 

 is atrophied. This peculiarity can now be explained by the fact that 

 the animal undergoes all its development as a parasite, and that 

 its life in the adult condition is nothing more than a short phase at 

 the beginning of which the sexual products are already ripe and 

 needing nothing further than to be disseminated. The animal has 

 no further need to assimilate. In the same manner the innumerable 

 insects called entomophagous, Hymenoptera, and Diptera for the 

 most part, are laid in the interior of other species where they develop 

 as parasites and become free and perfect images. In the same man- 

 ner again, among the annelids we know a series of Eunicidae which 

 develop as internal parasites in other annelids, to arrive, in the 

 adult, at a free stage which is in no way degraded by parasitism. 

 For the present I shall limit myself to this brief outline, which shows 

 that we must not believe that the state of parasitism has been reached 

 by simple and uniform series of transformations. In each case there 

 are special determining factors, and the consequences of these factors 

 are no less special. 



Thus it is not evident that the very striking adaptations of para- 

 sites, any more than in the case of free forms, are explicable by the 

 simple Lamarckian mechanism that we thought at first. Here again 

 parasites have been able to react in accordance with the successive 

 steps of a vast orthogenesis. But nevertheless it is striking to see 

 parasitism bring about, in the most diverse groups, parallel modifi- 

 cations which can not be wholly independent of circumstances out- 

 side the organism itself. When a rhizocephalous crustacean finally 

 shows the asexual reproduction, which the work of F. A. Potts has 

 demonstrated in Peltogaster so&kdis and T hompsonia, 4 can it be 

 imagined that this final stage was a predestined evolution resulting 

 from mere internal factors of these organisms independent of ex- 

 ternal circumstances? Can we refuse to think that the progressive 

 action of parasitism, which in the general evolution of types can be 

 nothing more than a side issue, has nevertheless been an external fac- 

 tor of much importance, one which, many times in similar conditions 

 but in independent series, has led to results of the same kind ? 



Free organisms without doubt give rise to analogous problems, but 

 the case of parasitism is of a special nature. The establishment of 

 the fundamental types of the animal kingdom goes back, as we now 

 know, to a past which is practically inaccessible. The dawn of tan- 

 gible paleontology shows us evolution almost completed in its main 

 features. But the differentiation of parasites is a second evolution 

 consecutive to the first. We can not reasonably suppose that the 

 parasites with their intense anatomical, embryological, and physio- 



* Carnegie Institution, Publication No. 215, 1915. 



