EVOLUTION— PYCRAFT 239 



by the gi-adual passage into the newer, and final stage of life. Young 

 Crustacea, and young Mollusca, show precisely similar, and indeed, 

 often more striking changes in their "ontogeny." There is no need 

 to regard them as adult ancestral characters. 



There is no need to postulate irresponsible vagaries of the "germ- 

 plasm" to be weeded out by "natural selection", or sorted out and 

 increased or rejected by "genes" hidden away in the chromosomes, 

 and defying detection even by the highest powers of the microscope, 

 in attempting to account for the phases presented in ontogeny. 



As the orderly sequence of development proceeds, the germ cells 

 are in due course formed out of the nascent tissues, just as are the 

 ancillary structures necessary to their survival — the generative glands 

 and ducts, and all the other organs and tissues of the body. 



There seems to be no warrant for a belief in a special "germ- 

 plasm" potentially variable, and ultimately expressing that varia- 

 bility in the "somato-plasm", which, whatever changes it may 

 undergo in response to external influences, is unable to transmit 

 such changes, being capable only of producing the visible, animated 

 body. 



But if the term "germ-plasm" be accepted merely to dis- 

 tinguish the germ cells — ova and spermatozoa — which, as they de- 

 velop, after union, gradually build up again a body after the fashion 

 of its race, many of our diflBculties in studying problems of descent 

 will vanish. We do not, except in a very general sense, postulate 

 a "germ-plasm" in the case of the Protozoa. If we say Amoeba is 

 composed solely of "germ-plasm", or of "somato-plasm", we are 

 merely making a distinction without a difference. But if the sub- 

 stance which forms the body of the Protozoan can reproduce other 

 like bodies indefinitely, we need ask no more. For the Protozoa 

 display an infinite variety of forms, free-swimming and sedentary, 

 ciliated and flagellated, naked, and with a hard-skeleton, and show- 

 ing adjustments to astonishingly varied conditions, including para- 

 sitism. The Metazoa do no more. 



The passage from the Protozoa to the Metazoa is by no means an 

 abrupt one. Wlien, and where, did the distillation and isolation of 

 Weissman's "germ-plasm" take place? When does it expel from 

 itself the substance called the "somato-plasm" charged with build- 

 ing up a new body? And in what part of that developing body, 

 beginning with the first cleavage-plane, does it establish itself, in 

 the Metazoa, until all is ready for it to take up its final resting 

 place and to begin the formation of nev\' "germ-plasm"? 



Only some of the more specialized Protozoa form out of the sub- 

 stance of their bodies definite male and female germ cells, and on 

 these the final continuation of the race has come to depend, as in 



