June 22, 190G.] 



SCIENCE. 



951 



to respect and consideration than that of 

 Weismann. 



II. Vaughan claims that there is no experi- 

 mental evidence for the transmission of ac- 

 quired characters. This is not so, there is 

 evidence. For instance, the experiments of 

 Weismann with Polyommatus phlceas, quoted 

 by Vaughan, are evidence, when properly in- 

 terpreted. 



With reference to the latter, I have said:" 

 What the Lamarck-Darwinian theory maintains 

 is that external stimuli acting upon an individual 

 may produce changes in its characters, and that 

 these changes are transmissible, i. e., may reap- 

 pear in subsequent generations. But this is now 

 exactly the view of Weismann. To quote his 

 example, in the butterfly Polyommatus phlaeas, 

 increased temperature (external stimulus) effects 

 darker color (change of character), and Weis- 

 mann further believes that this character (dark 

 color) may reappear in subsequent generations in 

 consequence of the increase of temperature. 



For the hereditary transmission of such ac- 

 quired characters Weismann has his own the- 

 ory, hut this theory does not deal any more 

 with origin of transmissible variations, hut 

 is a theory of inheritance {I. c, p. 155). 



I think this settles the point: we see that 

 characters reappear in the offspring that have 

 been acquired by the parents. Observations 

 to this effect are known, and, furthermore, I 

 believe that all variations are due to external 

 stimuli, and that there are no variations due 

 to so-called inner causes alone. For there is 

 a grave logical error in the latter assumption 

 (I. c, p. 144) . The conception of spontaneous 

 variation implies that a certain class of causes 

 does not act in variation, namely, the causae 

 efiScientes. Now, every process in nature must 

 have three kinds of causes: causse materiales, 

 causas efficientes, and causK finales. The ex- 

 clusion of the second class, while only the first 

 and third are admitted, renders this assump- 

 tion illogical : we need a causa efiiciens, or 

 external stimulus. That is to say, no germinal 

 or spontaneous variation is thiiLkable, unless 

 there is an external stimulus. Each and every 

 variation must be consequent on an external 

 stimulus, which necessarily precedes it in time. 



^^Biolog. Centralblatt, 18, 1898, p. 153. 



An objection often made in cases where the 

 transmission of acquired characters seems 

 probable is that the acquired character again 

 disappears in subsequent generations, after 

 the external infiuence has ceased, that is to 

 say, that the variations revert to the original 

 form. Of course, this should happen. As I 

 understand the dynamic theory, its claim is 

 that external infiuences permanently change 

 organisms only when they remain permanent 

 in their action, and that it takes time, and, if 

 the expression is permitted, effort on the part 

 of the environment to render any change more 

 or less stable. But just this latter effect is 

 due to inheritance, and rei)eated inheritance 

 only is able to fi:s a character to such a degree, 

 that it in turn obtains the necessary inertia 

 to be classed with the stable, that is to say, 

 inherited, characters, which offer a certain re- 

 sistance to additional changes of environment. 

 In this respect, J. A. Allen's remarks are per- 

 tinent,'^ where he emphasizes the simultaneous 

 and permanent action of external conditions 

 upon large numhers of individuals. A change 

 in the external conditions must act upon a 

 multitude of animals, and they all must vary, 

 and if they are more or less uniform in or- 

 ganization, they must vary in the same or a 

 similar direction. This is the real starting 

 point for any transformation that is to become 

 permanent. I do not believe that in nature 

 single chance variations (due to unusual 

 stimuli acting but once) ever become the 

 parents of a similarly changed offspring, but 

 I think it is always a large number of speci- 

 mens, in fact practically all that live under 

 the changed environment that begin to vary: 

 the environment simply forces them to do so. 

 This fact, and we have evidence for it (see 

 Allen, Z. c), goes far to furnish direct proof 

 for the action of external stimuli in variation, 

 and the phrase ' pressure of environment ' in- 

 troduced by C. H. Merriam" for this fact, the 

 permanent and irresistible application of cer- 

 tain external forces upon a multitude of or- 

 ganisms, expresses this identical view. This 

 pressure, generally, does not stop again after 



'= Science, November 24, 1905, p. 667. 

 " ScnsNCE, February 16, 1906, p. 244. 



