FROM FREE-LIVING FORMS. 91 



beginnings are lost, as has been remarked above (p. 1), in the pheno- 

 mena of ordinary life, which latter evidently forms the starting-point 

 of parasitism, or in other words, parasites have, by accommodating 

 themselves to the conditions of a parasitic life, in course of time sprung 

 from creatures originally free. 



The mode of origin which we thus assert for these creatures is in 

 principle precisely the same as that which we also assert, irujonsonance 

 with the doctrine of descent, for the individual free-living forms, when 

 we maintain their development to have been brought about by means 

 of various influences, either directly from one and another, or from a 

 common original form. The manner of adaptation is of course dif- 

 ferent, inasmuch as in the case of free-living animals there is usually 

 a development of faculties which bring about a more extended and 

 complicated capacity, whereas parasites, on the contrary, have a cor- 

 respondingly limited relationship to the outer world, according to 

 the degree of their parasitism. It is only under the influence of ever- 

 changing surroundings, and in the full enjoyment of unembarrassed 

 activity, that an organism can develop itself in every respect and 

 fully form its capacities. Limitation of function is succeeded by 

 stunted growth, and this it is which gives to parasites at least to 

 stationary parasites their peculiar features. The organs and arrange- 

 ments which serve to act upon the outer world, and are excited by it, 

 disappear under the influence of a confined existence ; and by thorough- 

 going parasites this is the case often to such a degree, that the whole 

 organism, which at other times is so artistically formed, degenerates 

 into a simple tube, whose capabilities are almost entirely expended 

 in nutrition and generation. 1 



These influences of parasitic life are especially apparent in those 

 forms, the near relations of which lead a life either completely, or at 

 least to a great extent, free. The classical researches of Johann Miiller 2 

 have made us acquainted with a Mollusc (Entoconclia mirabilis) which, 

 in its young form, possesses the usual attributes of these animals, and 

 does not differ from related young forms any more than the latter do 

 from eacli other ; it lives also, for a time, in the ordinary free state, 



1 This view had already been advocated previously to the rise of the Darwinian theory. 

 In the case of Epizoa by Nitzsch (Magazin der Entomologie, Bd. iii., p. 261, 1818), and for 

 Entozoa by my uncle Fr. S. Leuckart (" Versuch einer naturgemiissen Eintheilung der 

 Helminthen :" Heidelberg, 1827). The latter says (loc. cit. p. 10), "The Helminths show a 

 manifold relationship and likeness to other orders and classes, but at the same time present 

 important differences from the related forms of animals, which, without doubt, have their 

 origin in the entirely different mode of life of the parasitic worms, and in their circum- 

 scribed and completely isolated abode." 



2 J. Miiller, " Ueber Synapta digitata und die Enstehung von Schnecken in Holo- 

 thurien :" Berlin, 1852. 



