386 ORGANIC GROWTH sec. 



stimuli are removed all organic form disappears. If only 

 the necessary warmth is wanting, tliere is an end of it. Life 

 itself, and with it all the forms in which it resides, are only 

 the effects of stimuli, and therefore the above objection is 

 entirely delusive. Indeed, the definition of species, genera, etc., 

 is rendered possible only by the characters produced by 

 external influences. Take away the latter, and with them 

 the characters conditioned by them, and we have no species, 

 genera, etc., no "kinships" left. It follows from this con- 

 sideration that, as already maintained, the question of the 

 formation of species is by no means so profoundly affected as 

 systematists, and, curiously enough, physiological botanists 

 suppose, by the fact that plants — as, for instance, species of 

 cereals — revert to an original form when the external condi- 

 tions which determine their peculiarities cease to act. 



We employ colours as distinguishing characters. Chloro- 

 phyll is a most important factor in the life of plants, and yet 

 in tlie absence of light the green colour disappears — it is not 

 inherited, but the materials which are its basis are inherited, 

 and these materials have been acquired. If we removed from 

 plants during their development the stimuli which act upon 

 them, without causing their death, so that we obtained 

 abnormal forms, these would still retain by inheritance 

 certain established properties which would cause them, when 

 the action of the stimuli was renewed, to develop only in the 

 definite directions which were natural to them. 



In all organisms definite tendencies of growth are inherited, 

 which alone constitute the specific qualities of the various 

 species. 



The stimulus of gravity having acted upon plants during 

 endless ages, so that roots were developed in a downward 

 direction, the parts wdiich became roots by this particular 

 kind of growth have necessarily developed a tissue of a 

 special character, a tissue wdiose cells tend to grow towards 



