460 FACTORS OF ORGANIC EVOLUTION. 



begonias (especially Begonia phyllomafiiaca), bryophyllum, some ferns, 

 and many otlier plants, are all expressions of the growth force wbicli 

 is a more or less constant internal power. This growth force may give 

 rise to more definite variations than impinging stimuli do; but the 

 growth force runs in definite directions because it, in its turn, is the 

 survival in a general process of elimination. Many of the characters 

 of plants which — for lack of better explanation — we are in the habit of 

 calling adaptive, are no doubt sirai^ly the result of the eruption of 

 tissue. Very likely some of the compounding of leaves, the pushing 

 out of some kinds of prickles, the duplication of tioral organs, and the 

 like are examples of this kind of variation. We know that the char- 

 acters of the external bark or cortex upon old tree trunks are the result 

 of the internal pressure in stretching and splitting it. This simply 

 shows how the growth force may originate 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. 



We have now considered two general types of forces or agencies 

 which start oft" variations in ijlants — purely external stimuli, and the 

 internal acquired energy of growth. There is still a third general fac- 

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

 for the existence of sex, as we now understand it, is to originate differ- 

 ences by means of the union of two parents into one off sirring. This 

 sexual mixing can not be considered to be an original cause of uulike- 

 nesses, however, since sex itself was at first a variation induced by 

 environment or other agencies, and its present perfection in higher 

 organisms is the result of the process of continuous survival in a 

 conflict of differences. 



The recent rise of Lamarckian views seems to have been largely the 

 result of an attempt to discover the vera causa of variations, Darwin's 

 hypothesis of natural selection assumes variability without inquiring 

 into its cause, and writers have therefore said that Darwin did not 

 attempt to account for the cause of variations. ISTothing can be further 

 from his views. Yet some of our most recent American writings upon 

 organic evolution repeat these statements. Cope, in his always admi- 

 rable Primary Factors of Organic Evolution, writes that "Darwin only 

 discussed variation after it came into being." Yet Darwin's very first 

 chapter in his Origin of Species contains a discussion of the "Causes 

 of variability," and the same subject is gone over in detail in " Variation 

 of animals and plants under domestication." Darwin repeatedly 

 refers the cause or origin of variation to •• changed conditions of life," 

 which is essentially the position maintained by the Lamarckians; and 

 he as strenuously combats those who hold that definite variation is an 



