i6 THE PRESIDENTIAL ADDRESS 



or reactive character, determined quantitatively by quantitative 

 conditions of the environment, but becomes fixed and incorporated 

 in the potential of the race, so as to persist when other quantitative 

 external conditions are substituted for those which originally 

 determined it.' 



The effect of Lamarck's laws on the hereditary transmission of 

 acquired characters would be this : ' A past of indefinite duration 

 is powerless to control the present, while the brief history of the 

 present can readily control the future.' 



After hearing a very condensed statement of conclusions so 

 essentially bound up with the progress of Organic Evolution, I feel 

 sure that you will wish to be reminded of Prof. Ewing's words which 

 followed the address at York : 



* Now is the winter of our discontent made glorious summer by 

 this Ray of Lankester.' 



Returning to the unreported discussion on the inheritance of 

 acquired characters at Manchester, I venture to bring forward 

 certain observations opposed to a belief in Lamarckian evolution by 

 means of inherited experience — observations which I then described 

 and have not known to be answered. In the relationship between 

 enemy and prey there is very commonly no opportunity for the 

 latter to learn by experience. The wonderfully elaborate adapta- 

 tions by which sedentary insects are hidden from enemies have been 

 evolved, not by experience of enemies but by avoidance of enemies. 

 In these examples, and they are numberless, we are driven to accept 

 Weismann's conclusion and with him to invoke ' the all-sufficiency 

 of Natural Selection.' When one of the twig-like caterpillars, of 

 which there are so many in this country, is detected by an insecti- 

 vorous bird it can do nothing and is devoured at once. Its one 

 defence is the astonishingly perfect resemblance to a twig of the 

 bush or tree on which it lives. It is firmly fixed and its weight also 

 supported by an almost invisible thread so that it cannot escape as 

 many caterpillars do by dropping to the ground and sheltering in 

 the grass or among dead leaves. Its one chance of survival is to 

 gain so perfect a disguise that it will not be seen, and to attain this 

 end the adaptive devices are most elaborate and wonderful : its 

 twig-like shape and colours with the power of gradually adjusting 

 these so as to resemble the bark of the bush or tree on which the 

 parent moth laid the egg from which it came, even the power to 

 reproduce exactly the appearance of lichen, the rigid stick-like 

 attitude maintained during the hours of daylight. Finally there is 

 the evidence, recently obtained by Robert Carrick,!''' that the disguise 

 does protect ; for examples of one of these caterpillars, resting on 



i' Trans. Roy. Ent. Soc, Lond., vol. 85, part 4 (May, 1936), p. 131, 3 pis. 



