72 APPENDIX. 



stimulus suddenly disintegrated into simpler and more 

 stable compounds; through this disintegration they 

 evoke what is called the function of the disintegrat- 

 ing part for example, certain changes of form (mus- 

 cular contractions) or the excretion of the disinte- 

 grated products, etc. 



Now how is it possible that such unstable chemical 

 combinations, answering exactly to the needs of life, 

 could have arisen in such marvellous perfection if the 

 useful variations had not always been presented to the 

 ceaselessly working processes of selection? or, if the 

 constantly increasing adaptation to the constantly aug- 

 menting delicacy of operation of physiological sub- 

 stances had depended in its last resort on accidental 

 variations ? Hence, not only with regard to the "form" 

 of organs, but also with regard to the chemical and 

 physiological composition of their materials, we are 

 referred to the constant presence of appropriate varia- 

 tions. 



III. VARIATION AND MUTATION. 



I have still to add a few remarks on the subject 

 touched on in the footnote at page 31. The view 

 there referred to was discussed by Professor Scott be- 

 fore in an article published in the American Journal 

 of Science, Vol. XLVIIL, for November, 1894, en- 

 titled "On Variations and Mutations." Following the 

 precedent of Waagen and Neumayr, Scott sharply dis- 

 criminates between the inconstant vacillating varia- 

 tions which it is supposed [ ?] produce simultaneously 

 occurring "varieties," and "mutations," or the suc- 

 cessively evolved time- variations of a phylum, which, 

 constitute the stages of phyletic development. The 

 facts on which' this view is based are those already 



