OF PLANORBIS AT STEINHEIM. 23 



heavenly bodies, determining the morphological cycle of their successive species, then 

 types could only be persistent when they sprang from a point of origin near to the 

 source of the whole group to which they belong. The instances of persistent types 

 are all of this character, as far as I know them, and preeminently what Prof. Agassiz 

 would call embryonic types, when comparing them with the higher organisms of their 

 own group. 



The greatest contrast with these is afforded by the parasitic types, which vary from 

 those which are in the fully adult condition, still recognizable as Vermes, Crustacea, 

 Mollusca, etc., to those in which all the type characteristics are obliterated in the adults, 

 but still preserved in the young, and finally to those in which the type is difficult to 

 recognize at any period. Of the first classes there are many examples, of the second 

 fewer, and of the third very few illustrations. 



A large portion of the Epizoa among Crustacea, Entoconcha among Mollusca, Lingua- 

 tula among Arthropoda, are familiar illustrations of the second class ; while Gregarina 

 and Taenia, may possibly belong to the last, to which some of the males of several genera 

 of the Cirripedia make a near approach. 



It is difficult to escape from the conclusion, that the loss in the adults of the alimentary 

 canal and other parts and organs, which are foimd in the young of the second class, must 

 be attributed to the parisitic environment ; no other adequate cause whatever has as yet 

 been presented, and the losses take place in those parts which are especially aftected, and 

 become useless on account of the parasitic environment namely, the alimentary canal, 

 limbs, the shell, etc. In these cases, we can only account for the second and third classes, 

 by supposing that the differences arise from the greater or less completeness of the 

 parasitic mode of living, which time and habit have increased, imtil the environment 

 finally conquered the tendency of the growth, and of the laws of heredity to repeat in 

 the young the inherited characteristics of the type. How fast, or how slowly this was 

 accomplished in specific cases, is not the question, but simjily whether there were two 

 opposing forces at w'ork, one represented by heredity and growth, and the other 

 by physical causes or the environment, and I think this assumption is highly 

 probable. With regard to the third class of cases, it would be difficult to determine 

 whether they represented distinct types in the animal kingdom in some cases ; but the 

 gradation which is presented in the males of Cryptophialus among Cirripedia, where the 

 young are almost as degraded in organization as the adults, shows that the environment 

 has acted either throughout a long time or quickly and effectually, so as to destroy the 

 type characteristics even in the earlier stages. 



This would then be an extreme exhibition of the power possessed by physical causes 

 to alter the primitive organization, and in fact I do not see how we can otherwise account 

 for this result when we look at the results of modern research and the serious modifications 

 produced by the experiments of Schmankewitch upon Artemia and Branchipus among 

 Crustacea, of Carl Semper upon Lymneus among Mollusca, and of Siebold and others 

 upon Batrachians. 



But although this power be granted in the abstract, and as a corollary of all the 

 relations of animals to their environment, the fact remains that under all but the most 

 extraordinary conditions, animals maintain their type characteristics. They show this by 



