VARIATION. ADAPTATION. ORIGIN OF SPECIES 389 



Just as the first type of variations may be referred to the influence of 

 external factors, so also the second type, viz. adaptive variations, may be re- 

 garded as closely related [Detto, 1904]. By adaptive characters we understand 

 variations, many of which (p. 391, under 4) are induced by external factors, 

 changes which are characterized by some quite special feature, and which may 

 be considered as a purposeful reaction. Examples of such reactions are to be met 

 with everywhere, and many instances have already been given of these in Lectures 

 XXIV and XXV. The external factor in these cases acts as a stimulus, and 

 the reaction consists in a morphological or anatomical alteration, calculated to 

 render the plant more capable of making the best use possible of its surroundings, 

 or of protecting itself from injurious influences in its environment — as we may 

 term the sum total of the external factors which affect it. The capacity for so 

 adapting themselves is possessed by different plants in very varying degree, 

 and hence some are able to thrive in the most varied situations, whilst others 

 are injured, in some cases fatally, by extremely slight deviations from optimal 

 conditions. Even the most adaptive of plants have, however, their limitations, 

 for although amphibious plants can live in water as well as on land, there is 

 usually in the long run a certain minimum and a certain maximum degree of 

 dampness which may not be exceeded; in other words, amphibious plants cannot 

 on the one hand become aquatics nor on the other xerophytes. According to 

 Massart (i902),however,Po/ygowwm amphibium may assume a xerophilousform 

 and structure. Itis impossible to sayhow this widely distributed capacity to react 

 adaptivelyto external stimuli has arisen, but it must be assumed that external in- 

 fluences have been operating on the plant world in the same way and in no greater 

 variety, for thousands of years, and that, in the struggle for existence, those plants 

 whichfailed to react by adapting themselves in this manner succumbed. In other 

 words, the external stimuli at first resulted in reactions, some of which were 

 adaptive and some not, and by natural selection only those which responded 

 in a suitable manner remained in existence ; the offspring of these plants would 

 inherit such peculiarities and so gradually the power of adaptation would become 

 fixed by heredity. These are hypotheses, however, into which we need not go 

 any further. But we must emphasize the fact that by no means all stimuli 

 induce purposeful reactions. The gaU, for example, is of service only to the 

 insect, but is highly disadvantageous to the plant ; we must assume indeed, 

 by way of explanation, that the insect succeeded in deluding the plant, so that 

 instead of treating the insect as an enemy and an intruder it behaved towards it 

 as if it were a bit of itself. Under other conditions, also, we meet with non- 

 purposeful reactions, such as those which result from the application of an un- 

 wonted stimulus to which the plant is not subjected in a state of nature, and to 

 which it has had no opportunity of adapting itself. We know of no cases of 

 alteration in form which would serve as examples of such reactions, but illustra- 

 tions frequently occur, especially in the phenomena of movement, as when a 

 bacterium is attracted by ether, which is of no service to it, and is not repelled 

 by corrosive sublimate, which is fatal to it ; or when a root bends towards 

 light, and a tendril refuses to curve round a stick smeared with gelatine. We 

 may well believe — to select the last case — that the tendril would grasp such a 

 support if it had often the opportunity in nature of meeting with supports 

 possessing a gelatinous surface. 



As to the causes inducing this adaptive capacity perhaps the best sources 

 of information are studies on unusual and art^cial stimuli, especially as they 

 have been systematically investigated, and, further, since it is very doubtful 

 whether it is possible to study a ' natural ' stimulus of any kind which the plant 

 is not already acquainted with. Goebel (1898) has drawn attention to the 

 response given by Cardamine pratensis, which, according to Schenck (1884), 

 can produce a typically aquatic form, although it usually occurs on land ; but 



