THE NEOZOIC AGES. 263 



whom he writes is no more than that which a school- 

 girl learns in her few lessons in dissecting a buttercup 

 or daisy. It is easy for scientific triflers to exhibit 

 collections of plants in which species of different 

 genera and families are so similar in their leaves that a 

 cai'eless observer would mistake one for the other, or 

 to get up composite leaves in part of one species and 

 in part of another, and yet seeming the same, and in 

 this way to underrate the labours of painstaking 

 observers like Heer. But it is nevertheless true that 

 in any of these leaves, not only are there good charac- 

 ters by which they can be recognised, but that a 

 single breathing pore, or a single hair, or a few cells, 

 or a bit of epidermis not larger than a pin's head, 

 should enable any one who understands his business to 

 see as great differences as a merely superficial botanist 

 would see between the flower of a ranunculus and that 

 of a strawberry. Heer himself, and the same applies 

 to all other competent students of fossil plants, has 

 almost invariably found his determinations from mere 

 fragments of leaves confirmed when more character- 

 istic parts were afterwards discovered. It is high 

 time, in the interests of geology, that botanists should 

 learn that constancy and correlation of parts are laws 

 in the plant as well as in the animal ; and this they can 

 learn only by working more diligently with the micro- 

 scope. I would, however, go further than this, and 

 maintain that, in regard to some of the most im- 

 portant geological conclusions to be derived from 

 fossils, even the leaves of plants are vastly more 



