} Be 
[July 
Sow-Thistle, this juice is plainly seen; while in the Buttercup it | 
is acrid, and in the Blood-root it is red, yet in none of these 
does it seem to serve the purpose so well as in the Milkweeds. | 
It is not, either, confined to any one species, but is characteristic 
of most members of the order. The species number over thir- — 
teen hundred, are widely scattered over the world, and are most 
diversified in form and habit. Some of them are climbers, some 
creepers, some herbs of upright growth, some shrubs, and some 
_trees. Two features which are found in many of them are the 
milky juice and the peculiar flowers. In the common Milkweed 
of this country (Asclepias cornuti) these peculiarities are seen” 
better than in almost any other species. It is also well known 
that the plant will thrive in nearly every sort of situation; con- 
sequently it must possess some quality that gives it a better 
chance to grow than many other plants. The reason for its 
dominance is no other than the possession of its sticky, milky 
juice. 
The sap of plants, though varying in different species, serves. 
the same purpose in all. While primarily it is useful in one 
way, secondarily it may be beneficial in another; yet, not every 
608 The Milkweeds. 
possess a milky sap, only not so well developed as in the Milk 
weeds, it is only necessary to carry the imagination back to 
progenitor which possessed the slightest traces of this, and there 
is sufficient reason to look for its future perfection. In the fierce 
struggle which goes on constantly among plants, anything which 
is advantageous in the slightest degree is sure to be preserved; 
and as it is transmitted to the progeny, and by them sent 01 : 
downwards, it will improve at every step. If, for example, the 
_ plant becomes disagreeable to the animal grazing in its vicini 
