228 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



not, at once reverted to the O. spinosa if I had re- 

 planted it on the poor soil from which I took it. It 

 seems, therefore, to be a very hazardous and fallacious 

 method of testing the value of specific or other char- 

 acters by cultivation. A wild plant may or may not 

 change at once. Thus the carrot, Daucus carota L., 

 proved refractory with Buckman, but not with Vilmo- 

 rin, who converted this annual into a hereditary bien- 

 nial by sowing the seed late in the season, till the 

 character of flowering in the second season became 

 fixed." 



The prevalence of spinous plants in dry and desert 

 regions has often been described. ^ The same is true of 

 reptiles, although spines appear on some species in 

 fertile regions. Spines of plants are believed to be 

 twigs, petioles, leaves, etc., partially aborted under the 

 influence of drought, or the absence of the water neces- 

 sary to the tissues of the parts in question. Wallace 

 points out, however, that there are spinous plants in 

 humid climates, citing the Gleditschia (honey locust) 

 as an example. The spines of such plants may be sur- 

 vivals of periods of drought in previous geologic ages. 

 Or desiccation of certain parts of a plant might be a 

 form of abortion of those parts, a phenomenon which 

 is confined to no region, and is evidently due to causes 

 other than drought in some cases. Henslow (/. c.) 

 says : "They [spines] originate, I maintain, as a mere 

 accidental and inevitable result of the arrest of the 

 organ in question, such arrest being mainly due to 

 drought." 



One of the best expositions of the influence of the 

 physical characters of the environment on the struc- 

 ture of animals is to be found in Semper's work, Ani- 



1 Natural Science^ 1894, September, p. 179. 



