188 



TUE GENESIS OF SPECIES 



[Chap. 



rccij^itnr, rccipitur ad modum recijncntis, as the same rays of 

 light ^vliicli bleach a piece of silk, blacken nitrate of silver. 

 If, therefore, we attribute the forms of organisms to the 

 action of external conditions, i.e. of incident forces on their 

 modifiable structure, we give but a partial account of the 

 matter, removing a step back, as it were, the action of the 

 internal condition, power, or force which must be conceived 

 as occasioning such ready modifiability. But indeed it is 

 not at all easy to see how the influence of the surface of 

 the ground, or any conceivable condition or force, can pro- 

 duce the difference which exists between the ventral and 

 dorsal shields of the carapace of a tortoise, or by what 

 differences of merely external causes the ovaries of the two 

 sides of the body can be made equal in a bat and unequal 

 in a bird. 



AN ECHINUS, OR SEA-URCHIN. 



(The spines removed from one-half.) 



There is, on the other hand, an a j^'^'iori reason wliy we 

 should expect to find tliat the symmetrical forms of all 

 animals are due to internal causes. Tliis reason is the 

 fact that the symmetrical forms of minerals are un- 

 doubtedly due to such causes. It is unnecessary here 



