156 GYMNOSPERMS 



egg nucleus. Extremely rapid changes may take place in reproduc- 

 tive structures without any noticeable changes in the leaves. Most 

 of the apples on the market today were unknown 30 years ago; but 

 there has been little corresponding change in the leaves. 



Since this is true, there is no occasion for surprise if the leaf of the 

 true ferns is carried over into the Cycadofilicales, with little or no 

 change, while the reproductive structures become so modified that 

 they form the basis for a great phylum. 



As we have already remarked, leaves, stems, and roots might orig- 

 inate independently in different groups. Nearly all botanists beheve 

 that the land flora had an aquatic ancestry, and no one doubts that 

 algae preceded vascular plants. When algae transmigrated to the 

 land, it became necessary to develop protective, conductive, and 

 supporting structures, which had not been produced while the plant 

 was surrounded by water. By mutation, by smaller variations, by 

 natural selection, or in some other way, an efficient conducting sys- 

 tem was developed. 



Later, more algae may have transmigrated to the land, and, hav- 

 ing similar conditions to contend with, may have made similar re- 

 sponses and developed a vascular system much like that of the pre- 

 vious transmigrants. 



Because two systems of this sort resemble each other, it has been 

 assumed that one of them transmitted it to the other. Paleobota- 

 nists are dependent, to a great extent, upon the evidence of vascular 

 anatomy. Resemblances may be due to heredity, but it is possible 

 that some of the similar structures may owe their similarity to en- 

 vironment. 



And so there is a logical possibility that the leaves of Filicales and 

 Cycadofilicales may owe their similarity to similarity of conditions. 

 But we doubt whether details in venation and margins would be so 

 identical if developed in this way. There are such things as environ- 

 mental anatomy and hereditary anatomy. The former is more quan- 

 titative than qualitative and much more subject to change. If en- 

 vironmental anatomy were as powerful as hereditary, corn seeds and 

 pumpkin seeds planted in the same hill should not produce such dif- 

 ferent plants. 



We believe that the striking similarity of the leaves of Filicales 



