150 MAMMALIAN DESCENT. [Lzcr. VI. 



to start a Whale with a Whale that shall be mother 

 of all the Whales that may come in that line. The 

 embryo of that small sapper and miner, the Mole, 

 may be traced through a dozen stages, each of which is 

 the temporary equivalent of what might have been the 

 permanent form of some type of Vertebrate. That is, 

 these may be traced, as answering to so many platforms 

 or levels of life ; none of these, however, in this case, 

 is fitted for free individual existence except the last ; 

 the others are passed through, not stopped at. Yet in 

 the lower types of the Vertebrata we know that steps 

 and stages in the life-history of a type may be fitted for 

 free existence, by the development of temporary organs ; 

 in these we have true metamorphosis. But the rapid 

 change of parts that are of no use to the individual is 

 of the greatest interest to the biologist, who sees in them 

 the writings of an old history, the records of lost peoples. 

 If this be the true interpretation of these changes, and 

 if, as is very probable, the Mammalia existed, even in 

 the times that gave us the Primary fossiliferous rocks, 

 species, genera, families, and orders of Mammalia may 

 have come in, and gone out, in countless numbers, 

 before the appearance of the types with which we are 

 now familiar. 



My work at the Mole begins with an embryo, which, 

 in its curled-up form, is scarcely larger than a mustard 

 seed (see fig. 1, p. 15). In this stage the various organs 

 are developed, in rudiment at least, and the general form 



