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. 
502. 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 Hepaticae, 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. 
