1S96.] *^«-' [Bailey. 



the members of plants are simply outgrowths resulting from this growth- 

 pressure, or as Bower significantly speaks of them ("A Theory of the 

 Strobilus in Archegoniate Plants," Annals of Botany, viii, 358, 359), 

 the result of an "eruptive process." The pushing out of shoots from any 

 part of the plant body, upon occasion, the normal production of adventi- 

 tious plantlets upon the stems and leaves of some begonias (especially 

 Begonia phyllomaniaca), bryophyllum, some ferns, and many other plants, 

 are all expressions of the growth-force which is a more or less constant in- 

 ternal power. This growth-force may give rise to more definite variations 

 than impinging stimuli do ; but the growth- force runs in definite direc- 

 tions because it, in its turn, is the survival in a general process of elimi- 

 nation. Many of the characters of plants which — for lack of better ex- 

 planation — we are in the habit of calling adaptive, are no doubt simply 

 the result of eruption of tissue. Very likely some of the compounding 

 of leaves, the pushing out of some kinds of prickles, the duplication of 

 floral organs, and the like, are examples of this kind of variation. We 

 know that the characters 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 ofl" variations in plants — purely external stimuli, and the internal 

 acquired energy of growth. There is still a third general factor, cross- 

 ing, or, as Eimer writes it, "sexual mixing." The very reason for the 

 existence of sex, as we now understand it, is to originate diflerences l>y 

 means of the union of two parents into one offspring. This sexual mix- 

 ing cannot be considered to be an original cause of unlikenesses, 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. Nothing can be farther from his views. 

 Yet some of our most recent American writings upon organic evolution 

 repeat these statements. Cope, in his always admirable Primary Factors 

 of Organic hvolution, writes that " Darwin only discussed variation after 

 it came into being." Yet Darwin's very first chapter in his Origin of 

 Species contains adiscusbion of the "Causes of Variability," and the same 

 subject is gone over in detail in Variation of Animals and Plants Under 



