298 JENNINGS: CHANGES IN HEREDITARY CHARACTERS 



matter of selection by external conditions; many diverse stocks 

 were present, on an equal footing; some were destroyed, others 

 were not. 



What ground then is there for saying that the development of 

 given characters followed a definite course, as if predetermined? 

 The conditions described are exactly what we should require 

 to find if in past ages there were many varied stocks, some of 

 which were preserved by the action of natural selection. Looking 

 back over the series from a later age, we are bound of course to 

 find it a continuous development. If the same characteristics 

 were favorable in successive ages — and there is no reason why 

 they should not be so — then the same sorts of variations would 

 be preserved in those successive ages; a line of development once 

 begun would be continued. And if the same sort of characters 

 are favorable ones in different branches of a family, then similar 

 characters may well arise and follow a similar course of develop- 

 ment, in the diverse branches, as Osborn states they do. But 

 at the same time many other heritable variations arise, that are 

 not in the line of progress, and hence are not preserved through 

 selection; these are precisely the "varieties" described by the 

 paleontologists; the diverse races that I have described in 

 Difflugia and Paramecium, and that are found to exist in all 

 organisms. The conditions described by the paleontologists 

 support strongly the theory of evolution by gradual change, but 

 I cannot see that they tend to establish the view that variations 

 show a tendency to follow a definite course, as if predetermined. 

 The paleontologists appear rather to report precisely the con- 

 ditions which we are bound to find if evolution occurs through 

 the guidance of natural selection operating on a great number 

 of diverse variations, the typical Darwinian scheme. 



There is one other point which I wish I had time to take up, 

 but I have not. I will merely attempt to state in a few words 

 my impression of it. This is the point made by Bateson (1914) 

 in his Presidential Address before the British Association, and 

 farther developed by Davenport (1916) in a recent paper: the 

 proposition, namely, that since practically all observed variations 

 are cases of loss and disintegration, we are driven to suppose 



