May 4, 1906.] 



SCIENCE. 



687 



duced no direct evidence to sustain his 

 contention. What he did, was to show 

 that a body of facts were in harmony with 

 the assumption that acquired characteris- 

 tics were inheritable, but his facts were as 

 fully in accord with the assumption of 

 Professor Poulton. As Professor Poulton 

 had a certain amount of experimental evi- 

 dence and Professor Hyatt had none, I 

 thought that Professor Poulton had the 

 better of the argument, but he did not show 

 that acquired characters could not be in- 

 herited. 



Since hearing the discussion between 

 those eminent men, I have made an effort 

 to go over the arguments for and against 

 the 'Theory of Direct Causation.' I have 

 read much or most of what Spencer, Cope, 

 Hyatt and Dall have written on the subject. 

 Dall has stated the proposition fairly and 

 unequivocally. The facts that they pre- 

 sent can be explained on their fundamental 

 assumption, but they produce no direct 

 evidence that that assumption is correct. 

 Nageli, von Wettstein and Strasburger 

 represent in botany what Cope, Hyatt and 

 Dall represent in zoology. As has already 

 been stated, de Vries has collated a mass 

 of evidence, all of which is against the 

 views held by Nageli among the botanists. 

 The positive evidence is against the 'dy- 

 namic theory' or the 'theory of direct 

 causation. ' 



But I wish to repeat the words of Dall : 



If the dynamical evolutionist brings forward an 

 hypothesis which explains the facts of nature 

 without violence to sound reasoning, that hypoth- 

 esis is entitled to respect and consideration until 

 some better one is proposed or some satiating error 

 detected. 



It should also be remarked, that while 

 Weismann denies the inheritance of func- 

 tional variation, causing atrophy or hyper- 

 trophy of a part, he admits that climate 

 may produce hereditary changes by acting 

 on the germ-plasm. He, however, does not 



commit himself to the belief that such does 

 occur. 



I should like to enter into a full discus- 

 sion of the effects of temperature on the 

 pupa of Polyommatus phlceas in influ- 

 encing the color of the wings of the adult, 

 but time does not permit. However, ac- 

 cording to Weismann there is a critical 

 period, after which the raising or lowering 

 of the temperature does not affect the wing 

 color of the adult. Weismann has pointed 

 out that in southern Europe the golden- 

 winged spring brood is derived from the 

 pups of the dark-winged siunmer brood; 

 while the dark-winged summer brood is de- 

 rived from the golden-winged spring brood. 

 An increase in temperature does affect the 

 wing coloration, but there is no evidence 

 to show that a permanent hereditary change 

 is wrought. In fact the evidence is con- 

 trary to such an assumption. The period 

 of sensitiveness to temperature in these 

 butterflies is comparable to the period of 

 sensitiveness discovered by de Vries in de- 

 veloping polycephalic poppies. Although 

 the polycephalic poppies fluctuate in the 

 number of converted stamens from almost 

 to 150 and over, de Vries found no in- 

 stance of heads without indications of pis- 

 tillody of the stamens, and in no instance 

 were all the stamens transformed into pis- 

 tils. The relative number of converted 

 stamens, however, is largely determined by 

 physical conditions; abundance of plant 

 food and a sunny exposure are essential for 

 the best results. The point of similarity 

 between these experiments is that de Vries 

 discovered in developing poppies and Weis- 

 mann in developing Polyommatus a period 

 of sensitiveness to external conditions. 

 After this period is passed, varying phys- 

 ical conditions do not affect the fully de- 

 veloped adult. 



Weismann 's butterflies belong in de 

 Vries 's category of 'ever-sporting varie- 



