35 
mot undergone any changes within 4,000 years. He concludes 
that if man were a labile organism he would have undergone some 
appreciable change within that period. Temporary lability may 
occur, but there is a continual tendency toward the stable forms, 
or an optimum of adaptability. This peculiarly dogmatic argu- 
ment has about it a certain cadaveric odor. It is in sharp con- 
trast to the author’s usually scientifically rational arguments. 
The discussion. of the lichen-thallus is next taken up. Sachs * 
maintains that the flatness of the lichen-thallus (foliose) is due to 
the direct influence of sunlight (photomorphosis). Reinke points 
out that this can not be the cause, since there are numerous lichens 
with fruticose thalli. Wherever or in what form the lichen-thallus 
occurs, it indicates a special morphological adaptation to favor the 
function of assimilation. If light and chlorophyll were the only 
factors concerned, then all assimilating organs would have the 
Same structure, since constant factors must produce constant 
results, 
Since lichens. undoubtedly have a polyphyletic origin we have 
a striking illustration of the fact that identical morphological 
adaptations may be produced in phylogenetically distinct series, 
It demonstrates that a plant which is most suitably adapted to its 
€nvironment constitutes a stable form, toward which the less per- 
fect forms of the series tend. The author concludes that sexuality 
is the cause of variation; therefore, lichens which are propagated 
by soredia alone must have reached the highest degree of stability. 
The phylogenetic development in lichens must have been the re- 
sult of reproduction from spores and gonidia. If a lichen in the 
Course of its phylogenetic development becomes associated with a 
new alga it is changed from a stable form to a labile form. It 
May also be possible that one and the same fungus may unite with 
different algae (forms of Cora, Sticta, Stictina, and Solarina). This 
Subject, however, requires further study. Change of algae has al- 
teady been observed by Forssell.¢ According to Bonnier,{ 
*Sachs, J. 
Flora, 78: 21 5-243. 1894. 
_ $ Bonnier, G. Germination des Lichens sur les protonémas des — Bats < 
a Gen. de Bot. r: 165-169. 1889. < Gi ee 
t Forssell, K. B,J. Lichenologische Untersuchungen. Flora, 64: 1-8, 33-46. 
