88 THE ORIGIN OF SPECIES 



toes; pigeons with short beaks have small feet, and those 

 fwith long beaks large feet. Hence if man goes on select- 

 Zing, and thus augmenting, any peculiarity, he will almost 

 certainly modify unintentionally other parts of the struc- 

 ture, owing to the mysterious laws of correlation. 



The results of the various, unknown, or l)ut dimly 

 understood laws of variation are infinitely complex and 

 diversified. It is well worth while carefully to study the 

 several treatises on some of our old cultivated plants, 

 as on the hyacinth, potato, even the dahlia, etc. ; and it 

 is really surprising to note the endless points of structure 

 and constitution in which the varieties and sub-varieties 

 difiEer slightly from each other. The whole organization 

 seems to have become plastic, and departs in a slight 

 degree from that of the parental type. 



Any variation which is not inherited is unimportant 

 for us. But the number and diversity of inheritable 

 deviations of structure, both those of slight and those 

 of considerable physiological importance, are endless. Dr. 

 Prosper Lucas's treatise, in two large volumes, is the 

 fullest and the best on this subject. No breeder doubts 

 how strong is the tendency to inheritance; that like pro- 

 duces like is his fundamental belief: doubts have been 

 thrown on this principle only by theoretical writers. 

 When any deviation of structure often appears, and we 

 see it in the father and child, we cannot tell whether it 

 may not be due to the same cause having acted on both; 

 but when among individuals, apparently exposed to the 

 same conditions, any very rare deviation, due to some 

 extraordinary combination of circumstances, appears in 

 the parent — say, once among several million individuals 

 — and it reappears in the child, the mere doctrine of 



