THE ARGUMENTS FROM DISTRIBUTION. 397 



those strange obscurations of one type by the externals of 

 another type. When we see land-birds occasionally feeding 

 by the water- side, and then learn that one of them, the water- 

 ouzel, an " anomalous member of the strictly terrestrial 

 thrush family, wholly subsists by diving — grasping the stones 

 with its feet and using its wings under water " — we are en- 

 abled to comprehend how, under pressure of population, 

 aquatic habits may be acquired by creatures organized for 

 aerial life ; and how there may eventually arise an ornithic 

 type, in which the traits of the bird are very much disguised. 

 Finding among mammals, some that in search of prey or 

 shelter, have taken to the water in various degrees, we shall 

 cease to be perplexed on discovering the mammalian structure 

 hidden under a fish-like form, as it is in the Cetacea. Grant 

 ^hat there has ever been going on that re- distribution of 

 organisms, which we see still resulting from their intrusions 

 on one another's areas, media, and modes of life ; and we 

 have an explanation of those multitudinous cases in which 

 homologies of structure are complicated with analogies. And 

 while it accounts for the occurrence in one medium of or- 

 ganic types fundamentally organized for another medium, 

 the doctrine of evolution accounts also for the accompany- 

 ing unfitnesses. Either the seal has descended from some 

 mammal which little by little became aquatic in its habits, 

 in which case the structure of its hind linibs has a mean- 

 ing ; or else it was specially framed for its present habi- 

 tat, in which case the structure of its hind linibs is incom- 

 prehensible. 



§ 140. The facts respecting distribution in Time, which 

 have more than any others been cited both in proof and in 

 disproof of evolution, are too fragmentary to be conclusive 

 either way. "Were the geological record complete, or did it, 

 as both Uniformitarians and Progressionists have habitually 

 assumed, give us traces of the earliest organic forms ; the 

 evidence hence derived, for or against, would have had more 



