2 4 o THE POPULAR SCIENCE MONTHLY 



velopment is an inference from innumerable facts and se.vies of phe- 

 nomena, all of which are bound up together and rendered intelligible 

 by the theory of common descent. We therefore find that the founders 

 of theories of evolution turn to individual development as the court 

 of last resort, as the place where evolution may be detected in actual 

 process. For here is found the link that binds successive generations, 

 here variations arise, whether they be mutations or of the ordinary 

 fluctuating kind, whether they be germinal or acquired; here in the 

 individual life history the Lamarckian must look for the reflection of 

 the experiences of the individual back upon the germ; here the ad- 

 herents of orthogenesis must find their crucial evidence. 



In his theory of natural selection Darwin accepted as given the 

 data of individual development. But he saw clearly that the funda- 

 mental phenomena of heredity and variation had their seat in the 

 individual development, and he experienced the need of framing a 

 conception that would bind together the phenomena of hybridization, 

 the various forms of variation, atavism, telegony, regeneration, inheri- 

 tance of acquired characters and the like ; and in his volumes on " Ani- 

 mals and Plants under Domestication " he framed the provisional 

 hypothesis of pangenesis to include them all. I shall not attempt to 

 present the details of this theory, but I ma}'' be permitted to say that, 

 as a matter of logical arrangement of the assumed data, under the 

 circumstances of existing biological conceptions and of the state of 

 knowledge of the time, the theory was well worthy of its illustrious 

 founder. In its way, it was as original as the theory of natural selec- 

 tion, though some of its fundamental ideas had certainly been antici- 

 pated by previous writers. 



ISTor shall I attempt a critical estimate of the value of the theory 

 in the history of science; but I may be permitted to call attention to 

 certain features. In the first place, the theory was overburdened with 

 certain unnecessary conceptions such as inheritance of acquired char- 

 acters, atavism and telegony. The elimination of these conceptions 

 immensely simplifies the theory of individual development. In the 

 second place, it rested upon a fundamental conception, that of repre- 

 sentative particles, which amounts to a denial of the reality of indi- 

 vidual development. And in the third place, it assumed certain biolog- 

 ical processes — the existence of specific vital particles of ultramicro- 

 scopic dimensions, their radiation from parent cells, and their aggrega- 

 tion in other specific cells in a definite architectural pattern — for which 

 there is not only entire absence of evidence, but which are wholly 

 inconsistent with the known facts of cellular physiology. For these 

 reasons the theory had only provisional importance, as indeed Darwin 

 recognized in naming it the provisional hypothesis of pangenesis. 



The determinant hypothesis of Weismann, contained in his theory 



