1873.] 187 [McCrady. 



parasitic presence of insect ova; and to the cysts formed by the tis- 

 sues of animals, in consequence of the presence of entozoa. In fact, 

 all parasitism may be considered as bi-polar in its effects, though 

 often the effect on the one or the other party, is inappreciable. The 

 variability of animals and plants under domestication is due to the 

 addition of a form of parasitism upon man, which we call domestica- 

 tion, to their ordinary geo-parasitism; and we have no right to infer 

 from this, equal variability in their ordinary state, unless we can show 

 equal change and variability of parasitic conditions. It is change 

 of parasitism which induces change of structure; as we see in the 

 cases of the Lernseans, the Cirrhipedia, Dentalium, and the Gaster- 

 opod parasite of Synapta, among many others; and I believe that if 

 animals have, in the course of their genealogical history, undergone 

 great modifications of form, they have done so in consequence of great 

 change from one kind of parasitism to another widely different. The 

 consequent change of structure may have had a suddenness, compar- 

 able to that of the metamorphosis of an insect, or may have been 

 more gradual, according to the rate of change in the conditions of 

 parasitism. If, moreover, the change of parasitic conditions were 

 geographic, the consequent change of structure might include .a 

 whole fauna, involving the sifting action of " Natural Selection." 1 

 If the change embraced the whole earth, the structure of all animals 

 and plants would have been affected simultaneously. 



But especially it is to be noted, that while these changes of struc- 

 ture may be very extraordinary, they have their limit; So far as we 

 know, they never involve the potentials of the form changed, or the 

 ratios of these potentials to each other; that is what I may call the 

 Logos of the form. What is subject to modification is not this Logos, 

 but the relative degree in which its various potentials may be realized. 

 Especially the generative system, though subject to functional dis- 

 turbance by great changes of parasitic conditions, is not thereby 

 transmuted into a new generative type. No true sexual bar seems to 

 be thus produced between allied lineages. 



Now all Trematodes are zoo-parasitic, i. e., parasitic upon other ani- 

 mals, and we should expect in their case a wide departure from the 

 form of their nearest allies among merely geo-parasitic animals. I 

 do not hesitate therefore to compare the Trematodes with the Radiata 

 in general, but especially with Echinoderm larvae, more especially with 



1 Which is only one of the modus operandi of Organisis, and has nothing to do 

 with Evolution, or with the Origin of Species. 



