CH. XX] THE ORIGLN OF SPECIES 211 



effect, it should be between large numbers, whereas a plant can- 

 not on the average have more than six like itself around it. 

 ^ A consideration of the instances just given, or of still more 

 "important" differences, such as that between the embryo in 

 Dicotyledons and Monocotyledons, soon shows that infinitesimal 

 or fluctuating variation, though it occurs in every character of 

 every plant, is inconceivable as a means of effecting the great 

 differences that actually exist in the vegetable kingdom. In- 

 finitesimal variations would at once be lost by crossing with 

 their surrounding unmodified neighbours, and only if all were 

 modified in the same direction by the action of some definite 

 cause, e.g. the environment, would there be any likelihood of 

 the survival of the new form. And even then, it is hardly con- 

 ceivable that such changes as those instanced above should take 

 place in gradual stages. 



The y'lQVf that evolution is not directly guided by the need 

 of response to the actual necessities of plants, but' is a more 

 mechanical process, going on in compa^ati^'e indifference to 

 them, but with the disadvantageous ^'ariations at once thrown 

 out by natural selection, has been gaining in definition for many 

 years, especially since the rise of the study of genetics on Men- 

 delian lines; and the "hollow curve" observations, described 

 above, seem to show clearly that it has followed a definite more 

 or less determined course. 



Nothing but mutation, understanding by that a change of 

 measurable amount, hereditary, not connected by infinitesimal 

 stages with the more "tj'pical" form of the parent, and usually 

 differentiating, seems capable of explaining the bulk of the 

 specific, generic, and family characters that at present exist. 



Large mutations, often covering se^^eraI characters of a plant, 

 are by no means unknown, and go by the name of sports. Actual 

 observation has shown that a great number of these are here- 

 ditary, as in the well-known instance of the cockscomb. But 

 that such sports can give rise to new species has been strenuously 

 denied, because no instance of their undoubted survival in nature 

 has been recorded. But, so far as I am aware, no instance of the 

 formation of even a small variety by natural selection of in- 

 finitesimal or fluctuating variations has been recorded, and the 

 theory was accepted on account of its a priori probability. When 

 this difficulty is cast up to the supporters of natural selection, 

 they insist that there has not been time enough for the formation 

 of anything since man began to observe such things. But, as 



