CHAPTER II 



MORPHOLOGICAL VARIATION 



Morphological variation has reference to differences in form. If 

 two or more individuals possess the same structural characters and 

 if they have all attained the same relative development, then the 

 different individuals will differ only in size ; but if the characters 

 have not attained proportional development in the different indi- 

 viduals, then we shall note differences in form independent of 

 mere size. This is the simplest of all forms of variation and is 

 the one chiefly in the mind of older biologists, even leading to 

 the mistake of supposing that evolution is essentially a study in 

 morphology. 



The cause of morphological differences may lie in extremely 

 favorable or unfavorable conditions of life, especially as regards 

 food and climate, affecting different characters differently, or 

 they may arise from internal and constitutional causes, as in 

 giants and dwarfs, or in such extreme differences as in the mul- 

 berry leaves shown in Fig. 2. 



Instances of morphological variation are so common and so 

 easily noted as to scarcely require mention. Two apples are 

 exactly alike except that one is larger than the other. It is a 

 clear case of morphological variation. In this instance there is 

 no difference in the characters of the two individuals except that 

 cell division and growth have proceeded farther in one case than 

 in the other. Aside from this they are identical. If two stalks 

 of corn or if a number of pigs, sheep, cows, or horses are exactly 

 alike except as to size, then their differences are quantitative 

 only, and the effect is morphological merely. 



Again, two horses are of the same breed, that is, possess the 

 same characters, but their characters are not equally, that is, 

 proportionately, developed. In one the leg is longer, the hock 

 shorter, or the face wider between the eyes. These differences 



2.S 



