312 Lang . — The Prothallus 
on a priori grounds quite comprehensible that a genus of this 
group, which is known to have had a long geological history, 
might retain the spore-bearing generation but slightly modi- 
fied, and owe its survival almost entirely to the adaptations 
of the gametophyte to new conditions. The uniformity of 
the sporophyte in the genus Lycopodium suggests that this 
has actually been the case with regard to it, and, as has 
already been shown, the main differences between the prothalli 
can be put into relation with the conditions under which they 
live. These considerations appear to lead to some such view 
of the relationship of the species of Lycopodium , of which the 
life-history is known, as the following. 
In the L. cernuum type the primitive form of prothallus and 
young plant has been most completely retained. From forms 
like these others have been derived, in which the gametophyte 
is adapted to the saprophytic mode of life. In all of these 
the protocorm is undeveloped, and changes in form of the 
prothallus, &c., have taken place. In those in which the first 
leaf is able to reach the light, whether the prothallus grows in 
rotting wood or just below the surface of the soil, the chief 
difference in the embryology is the absence of a protocorm, 
and the foot remains small. Under this head come the 
L. Selago type and the L. Phlegmaria type which may be 
looked upon as somewhat similar independent derivations 
from the primitive form. Further, several types of prothallus 
adapted to life at a considerable depth below the surface have 
probably arisen from the primitive type independently of one 
another. In the embryos of these the foot has attained much 
greater development and the first leaves arise in pairs pro- 
tecting the apex. This includes the L. clavatum and L. com - 
planatum types which differ mainly in the structure of the 
prothallus. On this view several biological types would be 
recognized that may or may not characterize related groups 
of species. 
If the habit of the sporophytes, the prothalli of which are 
known, be now taken into consideration, a further confirmation 
of this view will be obtained. For of the twelve species the 
