100 THE ORIGIN OF PARASITES. 



Notwithstanding all differences, the constitution of the young form 

 points unquestionably in all these cases to the relations which obtain 

 between it and the Rhabditidse. The differences, moreover, are not so 

 great as they might seem at first sight, for, on the whole, they are 

 limited to the fact that the former condition of life, which was spread 

 over two generations, is now drawn together into one ; and this is a 

 phenomenon which we often meet with in animal life. I need only 

 remind the reader, by way of example, that in nearly related forms the 

 alternation of generations is often represented by a metamorphosis in 

 which the former preliminary generation is represented only by the 

 characters of the young form. 



But even these traces of a former independence may be more 

 or less completely lost, for we know that besides the species with 

 alternation of generations and metamorphosis, there are very often 

 others in which the state which was passed through by the former 

 as a free larva is relegated to the period spent in ovo ; so that 

 thus birth occurs at a stage of development which was previously 

 attained only in the free state. In such cases, of course, all those 

 properties remain latent which enabled the respective conditions to 

 obtain external manifestation ; and the form which in the previous 

 case was living and mature, is now indicated only in sucli faint outline 

 as is necessary for accomplishing the transit into a new stage of de- 

 velopment. Such being the case, we have, then, no right to make the 

 existence of a Ithabditis-like larva the exclusive criterion for the rela- 

 tions which obtain between the parasitic and free-living Nematodes. 

 By means of a continuous and ever-increasing adaptation to the con- 

 ditions of parasitism, this larval form may disappear, or, more correctly, 

 it may become unrecognisable in the processes of development in ovo. 

 Through such abbreviations of the history of development there may 

 then arise forms like Oxyuris, Trichoccphalus, Spiroptera, and others, 

 with embryos, which are not hatched in a free state, but remain in the 

 egg until they have found a host (p. 66). 



The differences which exist between these species must of course 

 be considered in exactly the same way as the specific differences 

 between free-living creatures. In every case the characters of an 

 animal are the factors which determine its mode of life; so that 

 if two animals deviate from each other, their capacities also vary, 

 and that in exact proportion to the degree in which they differ 

 Trwhocephalus and Spiroptera live under other conditions than Oxyuris. 

 Although they are all Entozoa, and even inhabit the same organs, yet 

 they differ in manner of locomotion, nutrition, and propagation, as well 

 as in other functions. It is these very differences which find expression 

 in the peculiarities of the external and internal structure, since the 



