34 THE EVOLUTION OF NATURAL SPECIES. 



variation, for every individual of such a species would be exactly like 

 every other; and there could be no progressive adaptation of its 

 powers to the changing environment, through natural selection or 

 any other process. 



(5) Must distinguish between the cause and the conditions of evolution. 



In view of these several conditions, we may safely attribute selec- 

 tion and the other laws of evolution resulting from adaptive action 

 to the organism as their cause, though we know that the environment 

 furnishes the sum of the conditions, under some combination of 

 which the cause must act. 



(6) Statistical proof of natural selection. 



H. M. Vernon, in his Variation in Animals and Plants, 1903, pp. 

 341-345, gives statistical proof, quoted from Dr. Bumpus,* that the 

 English sparrow in the Northern States of America, when suffering 

 from heavy storms of rain, snow, and sleet, loses by death more of 

 those presenting certain characters than of others. The conclusion 

 which Bumpus draws is that "Natural selection is most destructive 

 of those birds which have departed from the ideal type, and its 

 activity raises the general standard of excellence by favoring those 

 birds which approach the structural ideal." Vernon shows from the 

 figures given by Bumpus that though the longest and shortest birds 

 are most exposed to death from such a storm, the average length of 

 the birds that recovered from the effects of the storm after being 

 captured was 1.27 per cent less than that of the birds that perished. 

 The average weight of those that recovered was 2.38 per cent, less 

 than that of those that perished. If these averages represent blasto- 

 genic (or inherited) characters it would seem that the species is under- 

 going transformation through exposure to a climate to which it is 

 not yet fully adjusted. 



III. DISCONTINUITY OF SPECIES. 



In his volume entitled "Material for the Study of Variation," pub- 

 lished in 1884, Bateson points to the lack of correspondence between 

 the diversity of physical environments and the diversity of specific 

 forms as a feature unexplained by the theories of either Lamarck or 

 Darwin. On page 5 of his book we read : 



According to both theories [the Lamarckian and the Darwinian] specific diver- 

 sity of form is consequent upon diversity of environment, and diversity of environ- 

 ment is thus the ultimate measure of diversity of specific form. Here, then, we 

 meet the difficulty that diverse environments often shade into each other insen- 

 sibly and form a continuous series, whereas the specific forms of life which are 



* See Biol. Lectures, Wood's Hole, 1898, p. 211. 



