THE COURSE OF BIOLOGIC EVOLUTION. 29 



When Brongniart had made the discovery referred to he 

 changed his mind with regard to the plants of the coal meas- 

 ures, and ever afterward maintained that Sigillaria and 

 Calamodendron must be phanerogams, referring them to the 

 Cohiferae. This complete reversal of his former logical and 

 correct views was due to the preconceived opinion that 

 exogenous growth was necessarily correlated with coniferous 

 and dicotyledonous plants, as taught by De Candolle, and there 

 is still a French school of vegetable paleontologists, who, as dis- 

 ciples of Brongniart, continue to maintain that Sigillaria must 

 be placed in an entirely different class from Eepidodendron, and 

 Calamodendron from Calamites, and who are disposed to deny 

 the cryptogamic character of all forms possessing an exogenous 

 structure. 



Now the truth seems to be that in the process of develop- 

 ment in plants the exogenous structure has been attained in 

 varying degrees along several ascending lines, and that there is 

 a different kind of exogeny.iu the calamite, the lepidophvte, 

 the cycad, the conifer, and the dicotyledon, while some- 

 thing resembling exogeny has been shown to occur 

 in certain fossil ferns and in certain living monoco- 

 tyledons. Exogenous cryptogams probably no longer exist. 

 The reign of the cryptogam has come to an end. It 

 occurred in remote Carboniferous times when these plants 

 constituted the greater part of the earth's vegetation. It was 

 then that certain types of the L-ycopodiaeeae and Equisetaceae 

 became forest trees and were supported by exogenous trunks. 

 These types have long since disappeared according to the law 

 of the extinction of trunk lines of descent, and it is only the 

 earlier and simpler types that have come down to us according 

 to the law of the persistence of unspecialized types. The 

 filicine, equisetian, and lycopodian types continued to develop 



