26 THE SURVIVAL OP THE UNLIKE. [l. 



survival in a general process of elimination. Many of 

 the characters of plants which — for lack of better ex- 

 planation — we are in the habit of callinf? adaptive, are 

 no doubt simply the result of eruption of tissue. 

 Very likely some of the compounding of leaves, the 

 pushing out of soine kinds of priekles, the duplication 

 of floral organs, and the like, are examples of this kind 

 of variation. We know that the characters of the ex- 

 ternal bark or cortex upon old tree trunks are the re- 

 sult of the internal pressure in stretching and- splitting 

 it. This simply shows how growth -force may origi- 

 nate characters of taxonomic significance when it is 

 expressed as mere mechanical power acting upon tissue 

 of given anatomical structure. This power* of growth 

 is competent, I think, to originate many and important 

 variations in plants. I suppose my conception of it to 

 be essentially the same as that of the bathmism of Cope, 

 and the "theory of the organic growth" of Eimer. 

 Darwin seems to have come near to the same law when 

 he supposed that excess of food supply is the chief cause 

 of variation, for he thereby recognizes the correlation 

 of growth and variability; but in his conception, the 

 growth is the result of a direct and immediate external 

 stimulus, and not an internal acquired force. 



We have now considered two general types of forces 

 or agencies which start off variations in plants, — purely 

 external stimuli, and the internal acquired energy of 

 growth. There is still a third general factor, (grossing, 

 or, as Eimer writes it, "sexual mixing." The reason 

 for the very existence of sex, as we now understand 

 it, is to originate differences by means of the union of 

 two i)arents into one offspring. (See Essay II.) This 

 sexual mixing cannot be considered to be an original 



