214 THE MEANING OF EVOLUTION 
dient in the milk produced by the transformed gland. 
But there is another important constituent. When 
one does unaccustomed manual work the ordinary re- 
sult is the formation of a blister. The epidermis, or 
scarfskin, becomes detached from the dermis, or true 
skin, and the space between the two rapidly fills with 
the fluid portion of the blood, known as lymph. The 
fact that no blood vessels have been broken in this 
detachment results in there being no red corpuscles in 
this fluid. Wherever a cavity forms in the body 
lymph is liable to enter it. 
The milk glands of the mammals are modified oil: 
glands. The fluid which they now pour out is no 
longer exactly the old oil with the addition of the 
lymph. Undoubtedly in the past the first milk was 
more like this simple mixture. There seems no doubt 
that the breasts of to-day are the enlarged and modi- 
fied oil glands of earlier mammals. In one of the 
most primitive of our mammals the young simply 
lick certain bare spots on the surface of the mother’s 
abdomen. As higher forms arise there develops a 
smaller or larger mound with a distinct projection, 
about which the lips of the offspring can easily fasten. 
Lamarck would have said that the suction of the 
infant had produced such a mound, and that this had 
been transmitted to later offspring until it had grown 
to be the highly developed organ we now find, for in- 
