VIIL] HOMOLOGIES. 181 



Quicquid recipitur, recipitur ad modum recipientis, as the 

 same rays of light which 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 inci- 

 dent 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 modifi- 

 ability. 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 produce 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. 



There is, on the other hand, an a priori reason why we 

 should expect to find that the symmetrical forms of all ani- 

 mals are due to internal causes. This reason is the fact 



AN ECHINUS, OR SEA-URCHIN. 



(The spines removed from one-half.) 



that the symmetrical forms of minerals are undoubtedly due 

 to such causes. It is unnecessary here to do more than al- 

 lude to the beautiful and complex forms presented by inor- 



