84 The Mechanism of Evolution in Leptinotaesa 



two main divisions of organisms. The remarkable specialization and localiza- 

 tion of this material in higher animals has, no doubt, tended to restrict and 

 codify our conception of this material, to limit it unduly ; but in plants many 

 portions of the plant tissues, serving a purely somatic role, may on the advent of 

 a proper environmental reaction regulate, and from a purely somatic aspect 

 develop, divergently into the buds and the production of gametic masses or 

 germ-cells. In the development of the plant there had been specialization along 

 somatic lines, perhaps development of special features of structure and function, 

 but not complete loss of the capacity to recede from this position, to regulate, 

 return to an initial position, and produce buds with the proper specialized germ- 

 cells characteristic of the species. With higher animals this regulatory capacity 

 seems to have been lost and only that portion of the zygote that enters into the 

 formation of the gonads goes on into the next generation, while opinions differ 

 as to what really happens in lower animals. In all, however, there is the pro- 

 duction, by one means or another, of the necessary germinal masses or germinal 

 materials to carry on the specific combination of qualities and attributes of the 

 genetic lines of descent. 



The development by Weismann, Nageli, De Vries, and others of the concept of 

 a germ-plasm, idioplasm, and so on within the mass of the germ-cell, resident 

 perhaps within the chromatin of the nucleus, is entirely an anticipation of 

 nature and not even an interpretation. 



The independent position given to this germ-plasm in Weismann's hypothesis, 

 a microcosm within the germ-cell, the biophores being essentially living indi- 

 viduals, is a vitalistic conception not open to investigation, and, therefore, hardly 

 worth consideration until there is some exact experimental demonstration of its 

 reality. Whether the germ-plasm is a special portion of the reproductive cell 

 or whether the whole germ-cell is germ-plasm, is at present a matter of a priori 

 opinion, but it seems most naturalistic and reasonable to think of the whole 

 germ-cell as having germinal potentialities. 



The primordial germ-cells, through the division periods, seem to me to repre- 

 sent the real germ-plasm in its simplest condition, and are usually assumed to 

 have the same potential composition as the zygote in which they are located, and 

 no doubt are so, at least at the start. The organization and development of the 

 growth period are, however, it seems to me, the initiation of ontogeny, which 

 progresses for a certain time, but as a rule goes no further without fertilization 

 or the influence of some external agent, in order to set up the reactions neces- 

 sary to the series of ontogenetic reactions. Loeb's discoveries and those of his 

 followers with artificial parthenogenesis show most clearly that external physical 

 agents can provide the necessary impetus for the egg to develop without union 

 with some other germinal mass. These results that have come from the investi- 

 gations of artificial parthenogenesis show that development is not independent 

 of external motive forces, but is entirely dependent thereon, either through 

 normal fertilization or the action of purely physical forces, or both. 



Since the discovery of the crude aspects of the mechanism of fertilization, the 

 union of the pronuclei, we have assumed that the gametes were the same in their 

 hereditary potentialities ; but the facts of artificial parthenogenesis show clearly 

 that at least they are not the same in their development and it is not impossible 

 that they may also be different in their hereditary or germ-plasm potentialities. 



