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JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



off spring of individuals, exposed to certain conditions during several 

 generations, are modified in^the same manner. 



bidefinite variations are a much more common result of changed 

 conditions [i.e. under cultivation in complex soils] than d.e finite . . . 

 and have probably played a more important part in the formation of our 

 (iomesfic races, " but wof in nature. ; 



"Indirect Action is through the reproductive system." Darwin 

 here alludes to hybrids, and the difficulty in breeding wild animals in 

 confinement, as well as to tropical plants.* 



Darwin illustrates "direct action " . by geological changes: — 

 " According to nature of new conditions, so we might expect all or the 

 majority of organisms born under them to vary in some definite way 

 . . . the variations being the direct and necessary effects of causes 

 which we can see, and which can act on them . . . such new varieties 

 may then become adapted to those external agencies which act on 

 them." 



Again, " One problem , " he tells us in 1842, he had overlooked, 

 namely, ''the tendency in organic beings descended from the same 

 stock to diverge in character as they become modified . . . the solu- 

 tion, as I believe, is that the modified offspring . . . tend to become 

 adapted to many and highly diversified places in the economy of 

 nature." (My italics.) This " tendency " corresponds with the present 

 idea of " Eesponse." 



The above quotations show clearly that Darwin saw that "Here- 

 ditary Adaptive Structures " were the outcome of the Direct Action of 

 changed conditions of life. These are identical with new varietal or 

 specific characters. 



In October 1838 he read Malthus's " Essay on the Principle of 

 Population " (1798), and " being well prepared to appreciate the 

 struggle for existence . . . it at once struck me that under the circum- 

 stances favourable variations would tend to be preserved and unfavour- 

 able ones to be destroyed. The result of this would be the formation 

 of new species. Here, then, I had at last got a theory by which to 

 work." 



We are now passing to the stage when Darwin adopted " indefinite 

 variation " under cultivation, with natural selection," after reading 

 Malthus's work. 



The following is Malthus's contention. He argues that " a popula- 

 tion when unchecked doubles itself in every twenty-five years, or 

 increases in a geometrical ratio." But with regard to food in a 

 " limited territory," the output is obviously limited, and " considering 

 the present average state of the earth (a.d. 1798) the means of subsist- 

 ence, under circumstances the most favourable to human industry could 

 not possibly be made to increase faster than in an arithmetical ratio." 



What, then, will check population? "The immediate check may 



*Ori(jfin, <(•(*., sixth edition, p. 6. I cannot find any allusion to the origin 

 of iijiecies by "Indirect Action." 



