MORPHOLOGICAL VARIATION 27 



are all quantitative and morphological, but they influence form 

 rather than size as a whole, because their development is not 

 proportional one part with another. 



Variation seldom simple. Instances of the above kind are, 

 however, extremely rare. Variation is so common that other 

 differences generally accompany those of form. The two apples 

 may differ in color, flavor, or texture as well as in size, in which 

 case substantive variation has also occurred. One of the horses 

 may have an extra rib, one of the pigs a solid hoof, or one of 

 the sheep more fibers of wool to the square inch, in which case 

 meristic variation is present. The pulse of one of the horses 

 may be faster than that of the other, or the milk of one cow 

 may be richer in fat, showing functional deviation. 



And so it is in practice that two or more forms of variation 

 may be and likely will be found present in the same individual. 

 But however that may be, all differences in form or size are 

 regarded as morphological, no matter what other differences 

 may be found, and it is important that the student early form 

 the habit of distinguishing clearly between the different kinds 

 of variation present, even in the same individual. 



The limits of size. Every species has a general average of 

 size to which most individuals closely approximate. A few, how- 

 ever (giants), greatly exceed this size, and others (dwarfs) fall 

 far short of it. All investigators agree upon the conclusion that 

 this difference in size is due to the number and not the size 

 of individual cells ; in other words, size is dependent upon the 

 energy of cell division. 1 This energy is exceedingly active in 

 youth, gradually decreasing to zero at maturity, except as to 

 certain parts (reproductive organs, skin, and sometimes the 

 teeth and horns). In most species accident to a part will stim- 

 ulate cell division, leading to a more or less successful regenera- 

 tion. For the most part, however, cell division does not proceed 

 rapidly after maturity, and the limit of its activity is in general 

 the limit of growth. Giants therefore represent excessive cell 

 division above the normal, and dwarfs, arrested development, 

 an abnormally early cessation of cell division. 



1 Wilson, The Cell in Development, pp. 388-394. 



