SOME COMMON BOTANICAL ERRORS 145 



ress, and to inconsistencies requiring the most artful 

 dodging. The central point in this doctrine is the 

 belief in the comparative immutability of the nature of 

 the plant members or elements. All modern research, 

 however, is denying this belief and replacing it by the 

 opposite principle, viz. that difference of degree of de- 

 velopment passes over into difference of kind of struc- 

 ture, thus leading to the formation of new elements 

 or members which become centres of variation, modifi- 

 cation, adaptation upon their own account, and more or 

 less independently of their original nature. Thus the 

 ovary is composed of carpels, which originally were 

 spore-bearing leaves. Now, when an ovary must vary 

 adaptively to some new influence, it does not need to go 

 back to consult the rules governing its behavior when it 

 was a set of sporophylls, but it responds as a unit, as 

 an ovary ; it has itself become a member or element. 

 It is always necessary in morphology to keep plain the 

 difference between historical origin and present nature. 

 Historically, an American is an Englishman, but he 

 does not on that account now act or think as an Eng- 

 lishman ; he has a new character, he is an American. 

 So, historically, the ovary is a set of leaves ; but it does 

 not act like a set of leaves, but like what it now is, an 

 ovary. The placenta is another good example of this. 

 Generally it is said to represent the united edges 

 of infolded carpellary leaves, but one has to perform 

 complicated mental gymnastics to interpret all placentae 



