NATURAL SELECTION 221 
have, as is asserted, long or many horns; pigeons with feathered feet 
have skin between their outer toes; pigeons with short beaks have 
small feet, and those with long beaks large feet. Hence if man goes 
on selecting, and thus augmenting, any peculiarity, he will almost 
certainly modify unintentionally other parts of the structure, owing 
to the mysterious laws of correlation. 
DARWIN’S IDEA OF THE CAUSES RESPONSIBLE FOR THE ORIGIN OF DOMESTIC RACES 
To sum up on the origin of our domestic races of animals and 
plants. Changed conditions of life are of the highest importance in 
causing variability, both by acting directly on the organization, and 
indirectly by affecting the reproductive system. It is not probable 
that variability is an inherent and‘ necessary contingent, under all 
circumstances. The greater or less force of inheritance and reversion 
determine whether variations shall endure. Variabilityis governed by 
many unknown laws, of which correlated growth is probably the most 
important. Something, but how much we do not know, may be 
attributed to the definite action of the conditions of life. Some, per- 
haps a great, effect may be attributed to the increased use or disuse 
of parts. The final result is thus rendered infinitely complex. In 
some cases the intercrossing of aboriginally distinct species appears to 
have played an important part in the origin of our breeds. When 
several breeds have once been formed in any country, their occasional 
intercrossing, with the aid of selection, has, no doubt, largely aided in 
the formation of new sub-breeds; but the importance of crossing has 
been much exaggerated, both in regard to animals and to those plants ° 
which are propagated by seed. With plants which are temporarily 
propagated by cuttings, buds, etc., the importance of crossing is 
immense; for the cultivator may here disregard the extreme variability 
both of hybrids and of mongrels, and the sterility of hybrids; but 
plants not propagated by seed are of little importance to us, for their 
endurance is only temporary. Over all these causes of Change, the 
accumulative action of Selection, whether applied methodically and 
quickly, or unconsciously and slowly but more efficiently, seems to 
have been the predominant Power. 
DARWIN’S IDEA OF THE ORIGIN OF VARIETIES, SPECIES, AND GENERA IN NATURE 
Finally, varieties cannot be distinguished from species—except, 
first, by the discovery of intermediate linking forms; and, secondly, 
by a certain indefinite amount of difference between them; for two 
