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THE AMEBIC AX NATURALIST [Vol. XL1II 



Although perfectly, true in a physiological sense, this is 

 incorrect in this connection. Potentially the egg of one 

 animal is as different from that of another as their adult 

 forms, but morphologically they correspond. Morphology 

 is not concerned with the ' ' growth energies ' ' of organisms, 

 but only with their form and structure. A similar mis- 

 take was made by His in the quotation cited above, where 

 he takes for an illustration of his views an animal which 

 had lengthened its life over that of its ancestors. The 

 logical deduction from such an example under the re- 

 capitulation theory would be that the last form should 

 die at the end of each period, one, two, three years, etc., 

 in order to recapitulate its ancestry, rather than that 

 it lives one, two, three years to do so. The absurdity 

 of this lies in the fact that length of days is not a morpho- 

 logical character. The recapitulation theory has nothing 

 to do with physiology; it is purely a matter of mor- 

 phology. 



The degree of approximation between the young of 

 a higher form and the adult of a present-day lower 

 form of the same line depends upon the degree of spe- 

 cialization and divergence of the lower species from the 

 main path of descent. It is usually recognized that 

 most of the lowest and morphologically simplest organ- 

 isms are highly specialized for some particular mode 

 of life more or less different from the ancestral. This 

 specialization nearly always carries with it some struc- 

 tural adaptations, but these may not obscure the ances- 

 tral characters. Thus Mareliantia lias evolved a cham- 

 bered thallus highly differentiated, to adapt it at once 

 to an aquatic substratum and aerial life, but it still 

 retains a sporophyte perhaps very similar in some fea- 

 tures to that of the ancestors of the higher plants at 

 the liverwort stage. On the other hand, organisms are 

 occasionally found which give every indication of being 

 primitive. These are truly forms with arrested evolu- 

 tion. Renfrewia is an example; Anthoceros is another, 

 less free from specialization but contrasting strongly 



