252 CHEMISTRY OF PLANT LIFE 



Hence, the two terms, as we will use them here, may be defined 

 as follows: morphological adaptation is a change in the structural 

 character of the species in order that it may be better fitted to meet 

 the needs of the new conditions of growth; while physiological 

 adaptation is an acquired power to survive and develop under 

 abnormal conditions, which is not accompanied by any visible 

 change in the characteristic structure of the species. 



Both of these types of adjustment may be either hereditary 

 (or evolutionary), or spontaneous in their origin and development. 

 Changes which are evolutionary are fixed by heredity and become 

 definite habits of growth in the species. Their origin may be 

 explained in either one of two ways; namely, the so-called " in- 

 crease by use," and " the survival of the fittest." The hypothesis 

 of " increase by use," as an explanation of adaptations, is based 

 upon the well-known observation that, in animals, muscles and 

 other organs increase in volume as they are extensively used; and 

 the assumption of the application of this principle to the phe- 

 nomenon of adaptation supposes that the modification of any 

 given structure or composition is the result of the hereditary 

 accumulations of increased size resulting from use, or of atrophy 

 from disuse. The " survival of the fittest " theory supposes that 

 individuals of a species differ from each other by spontaneous vari- 

 ations, and that in the competitive struggle for existence those 

 forms which are best adapted to the environmental conditions 

 survive while the others perish. The contrast between these two 

 views is that the first holds that adaptation proceeds by develop- 

 ment, and the second that it proceeds by variation and elimination; 

 the first presupposes the existence in the organism of a mechanism 

 for response to changing conditions, and the second assumes that 

 there are chance variations followed by the death through compe- 

 tition of the forms which are not able to meet the needs of the 

 environment. 



Confusion arises whenever an attempt is made to apply either 

 of these theories to all kinds of adaptations. The idea of increase 

 by use can be applied with some satisfaction to certain morpholog- 

 ical adaptations in animal structure; and to such phenomena as the 

 increase in strength of the branches of fruit trees, either with or 

 without corresponding increase in size, as the load of fruit increases. 

 But it certainly cannot apply to color change in surface pigmenta- 

 tion of either animals or plants, which is one of the most common 



