Volume 1.2 Number 10 



The Plant World 



A Magazine of General Botany 

 OCTOBER, 1909 



INFLUENCE OF ARIDITY UPON THE EVOLUTIONARY 

 DEVELOPMENT OF PLANTS. 



By D. T. MacDougal. 



From every excursion which the biologist has made hitherto 

 into speculation as to the origination of living or self -generating 

 matter and its development into organisms, in which he has 

 called to his support extreme or unusual intensities of terrestrial 

 and atmospheric conditions, he has been ruthlessly recalled by 

 the geological historian with the reminder that the general 

 composition of the atmosphere, its pressure, the temperatures 

 and other conditions prevalent on the earth's surface were uni- 

 form and continuous with those now encountered and not widely 

 different in their total departure in any stage of the earth's de- 

 velopment in which life might have originated. 



Knowing full well that life did not always exist, that self- 

 generating matter, so far as our observations go, is not now 

 originating, we persistently return to the idea that the beginning 

 of life must have occurred at some stage of the earth's history 

 more favorable to such action than the present. 



THE ORIGIN OK LIFE. 



In the search for supporting ideas upon which to base 

 speculation, two conceptions serve as encouragement for a re- 

 newed attack upon this fascinating problem. One is embraced 

 by Chamberlin's planetesimal theory of the growth of the earth 

 and the attendant modification of surface conditions, which 

 necessarily showed a complex widely different from the present, 

 and the other is one growing in favor with physiologists to the 

 effect that the essential activities of living matter rest upon 

 catalysis and enzymatic processes, with the characteristic re- 



