THE ARGUMENTS FROM DISTRIBUTION. 485 



imposed by physical conditions, and in other cases voluntarily 

 commenced and slowly increased in the search after food; 

 we shall begin to understand how, in the course of evolution, 

 there have arisen strange obscurations of one type by the 

 externals of another type. When we see land-birds occa- 

 sionally 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 — grasp- 

 ing the stones with its feet and using its wings under water " 

 — we are enabled 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. On 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 and the Sirenia: especially on finding that in 

 the sea-lion and the seals there are transitional forms. Grant 

 that 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 organic 

 types fundamentally organized for another medium, the doc- 

 trine of evolution accounts also for the accompanying unfit- 

 nesses. 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 limbs has a meaning; or else it 

 was specially framed for its present habitat, in which case 

 the structure of its hind limbs is incomprehensible. 



§ 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 



