The Theory of Evolution 37 
handed down as a sort of entailed heirloom, but that the 
descendants have followed the same plan of structure as that 
of their ancestors, and have the brain enclosed in a covering 
of harder material, although this material may not have 
exactly the same form, or be made of the same substance in 
all cases. Furthermore while we may recognize that the 
cartilaginous skull of the shark is simpler in structure than 
that of the cartilaginous-bony skull of the frog, and that 
the skull of the frog is simpler than that of the rabbit, 
yet we should not be justified in stating, except in a 
metaphorical sense, that something has been added to the 
skull of the shark to make that of the frog, and some- 
thing to the latter to make that of the rabbit. On the con- 
trary, while something may have been added, and the plan 
made more complicated, the skull has also been changed 
throughout in every single part. 
There is another point of some importance to be taken 
into account in this connection; namely, that each new 
generation begins life as a single cell or egg. The egg 
does not contain any preformed adult structures that it 
hands down unaltered, but it is so constructed that, under 
constant conditions, the same, or nearly the same, kind of 
structure is produced. Should something affect the egg, 
we can imagine that it might form a new combination on 
the same general plan as that of the old, yet one that differed 
from the original in every detail of its structure. It is this 
idea, I believe, that lies at the base of the transmutation 
theory. On sbme such assumption as this, and on this 
alone, can we bring the theory of transmutation into har- 
mony with the facts of observation. 
What has been said in regard to individuals as a whole 
may be repeated also in respect to the study of the single 
organs. Selecting any one group of the animal or plant: 
kingdom, we find the same organ, or the same combination 
of organs present in whole groups of forms. We can often, 
