576 STRUCTURE AND LIFE HISTORIES 



of alternation. From this point of view two alternatives 

 are recognized: 



1. Either the fertilized egg and the haploid spore are 

 potentially unlike, and will therefore produce unlike plant 

 bodies, even under essentially similar environment, or 



2. Fertilized eggs and spores are potentially alike, but 

 produce unlike plant bodies as the result of the difference 

 in the environment in which they develop. 



The ontogenetic school accepts the latter alternative 

 as a working hypothesis, and regards the gametophytic 

 and sporophytic generations as essentially homologous. 

 The degree of homology which can actually be traced in 

 the vegetative structure of the two generations may vary 

 from substantial identity, as in Dictyota, to such wide 

 divergence that the tracing of homologies is quite out 

 of the question. In testing this hypothesis a crucial 

 experiment would be to obtain a gametophyte by arti- 

 ficially bringing a fertilized egg to mature development 

 outside of the archegonium and under the environment in 

 which the spores normally develop; or to obtain a sporo- 

 phyte by causing a spore to develop within the tissue of a 

 gametophyte, as the fertilized egg normally does. 



602. Hypothetical Ancestral Tree. From a compara- 

 tive study of both living and fossil forms some botanists 

 have been led to infer the common derivation of Filicales, 

 Equisetales, and Lycopodiales from the Hepaticse, and 

 probably through some form belonging to the Anthocero- 

 tales somewhat as shown in the following ancestral "tree" 

 (Fig. 410). It should be clearly understood that this 

 tree does not illustrate known facts, but only the hypoth- 

 eses which have been tentatively proposed by careful 

 students on the basis of known facts. 



