28 IOWA ACADEMY OF SCIENCES. 



was originally derived. In other words, the individual 

 was preformed in the germ, and its development was 

 therefore a process of unfolding. This idea was accepted 

 or independently proposed by a number of naturalists, 

 until finally it was put forth by Darwin in a carefully 

 elaborated form, with a wealth of illustration, as the "Pro- 

 visional Hypothesis of Pangenesis. " 



The theory assumes that all the cells of an organism 

 produce minute corpuscles, called gemmules, which after 

 their formation are scattered throughout the body. These 

 gemmules are produced not only in the adult organism, 

 but at all stages of development; since their production 

 depends on the physiological maturity of the cells from 

 which they are derived, not on that of the entire organism. 



These gemmules have the power of absorbing nourish- 

 ment and increasing in number by self-division. Tliey 

 have for one another an affinity which causes them to 

 come together in the reproductive glands, the aggregate of 

 all the different kinds collected there in the germ cells 

 constituting the material basis of heredity. Development 

 consists in the growth of a portion of each kind of gem- 

 mule into cells like those from which it originated. All 

 the gemmules in the germ cells, however, do not so 

 develop, but many of them, probably the greater part, 

 are passed on unchanged and may develop in a later gen- 

 eration; hence the phenomenon of reversion. Before their 

 aggregation in the germ cells, while still scattered through 

 the system, the gemmules may under certain conditions 

 develop; in this way the reproduction of lost parts may 

 be accounted for. 



Many objections to the theory of pangenesis, apart from 

 general objec4:ions to all preformation theories, have been 

 brought forward, but there is not here time to consider 

 them in detail. One or two of a somewhat general char- 

 acter may, however, receive brief mention. 



It is assumed that the gemmules are given off at all 

 periods of development, and hence a large share of them 

 must be derived from immature organs; and it would seem 

 as if there would be a constant tendency for each succes- 



