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 some 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 



