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somehow or other, ultimately to a modification of struc 
tures similar to that undergone by individuals. It is 
assumed that this will take place when imaginary cases 
are suggested to illustrate the way in which the trans 
mutation of species might be brought about by Natural 
Selection. 
We see actual illustrations of this principle in the 
different kinds of domesticated dogs, among which 
different habits of life are coexistent with different modifi 
cations of structure. In the case of the trotting horse of 
America, already referred to, there is a correlation, how 
ever caused, between the mode of progression and the 
modifications of structure which result therefrom. 
There are two points of difference between changed 
conditions and changed habits in modifying species, to 
which it may be well to draw attention. In the first place, 
the change of outward conditions acts upon an organism 
which is more or less passive, if responsive ; whereas in the 
case of habits the organism is the active coadjutor. This 
is altogether to the advantage of habit, considered as a 
transforming influence. But there is another respect in 
which habit is sometimes, though not always, at a com 
parative disadvantage. It does not follow, at least so far 
as our experience is concerned, that persons who have 
the same habits necessarily intermarry. The accomplished 
musician does not always marry the accomplished musician; 
the athletic youth does not necessarily marry an athletic 
maiden ; the poet does not generally marry a poet (the 
Brownings afford a splendid exception to the rule), nor 
a philosopher a philosopher. Hence the special aptitude 
of the one is qualified by the normal incapacity of the 
other, and the offspring cannot be expected to reproduce 
in all cases the attributes of the more gifted parent. But 
in some cases both parents follow the same calling and 
BB