610 PROCEEDINGS OF THE NATIONAL MUSEUM vol.87 



change from the ancestral condition. Such species are sometimes 

 called sports. Numerous examples are known. One of the best 

 known is that of the sudden appearance of hornless (mulley) cattle, 

 a mutation worthy of recognition as of specific rank. But the dis- 

 tinction between the gradual development and summation of small 

 divergencies and the sudden appearances of major differences is not 

 the only one of interest. The degree to which natural selection affects 

 the development of divergent organisms also is of importance. Speci- 

 ation of Opalinidae has two noteworthy features: First, species in 

 this family do not arise through the sudden appearance of markedly 

 divergent individuals. This is indicated by the fact that species often 

 so grade into one another as to make it well-nigh impossible to define 

 boundaries between species. Second, natural selection has been less 

 influential in the evolution of the Opahnidae than in the evolution of 

 very many families. This is evidenced by the same phenomena of 

 intergrading species, the struggle for existence not having destroyed 

 the intergrades, but all persisting in apparently equally favorable 

 relation to the environment. We see, then, that the Opalinidae 

 diverge by origin of slight differences and that the slightly divergent 

 forms, having appeared, are unusually free from the action of natural 

 selection. 



This freedom from control by natural selection is due to two factors: 

 First, to the fact just mentioned that the divergences arising are very 

 slight and so do not much, if at all, influence success in the struggle 

 for existence. The several slightly diverse forms are all equally suc- 

 cessful. The second factor is that the parasites live such secluded 

 lives in so uniform an environment that they escape the stress of life ; 

 also there is no diversity of environment to provide peculiar conditions 

 into which specially adapted organisms might fit. Such divergent 

 evolution, speciation, as has occurred in the Opalinidae is, therefore, 

 due less to natural selection and more to the nature of the animals 

 themselves than in most other families. In the Opalinidae the 

 internal factors of evolution are not prevented by environmental 

 influences from expressing themselves. The Opalinidae are what 

 they are through self-determination to an unwonted degree On this 

 account their character is self-revealing and not due to molding by 

 external influences. 



Of course, there is one great exception to this statement. The 

 Opalinidae are parasites, or, more properly, intestinal commensals, 

 and so much of their character as is in adaptation to life within the 

 intestine of a host is doubtless in response to this major condition of 

 their environment. They live bathed in predigested, nutritive fluid, 

 and probably in adaptation to this condition they have no mouth, 

 no digestive vacuoles, and, so far as we know, no digestive fluids, and 



