Spore Formation in Torreya californica. 141 
reached when the microspore as a whole, instead of the naked 
sperm, assumed the duty of journeying to the archegonium. Before 
that time the male prothallus, even if it was of little use, would 
probably have survived, as the possession of it would not have been 
an active disadvantage to the plant. But as soon as wind trans¬ 
ference came into play, lightness became a very important quality 
in the microspore, and every superfluous structure would be rigidly 
eliminated by natural selection. The pollen-grains would have to- 
be carried by the wind even' to secure 5e//-fertilisation, and the 
lighter the grain, the greater would be its chance of being conveyed 
to a distant tree and so effecting cross fertilisation and the pro¬ 
duction of a vigorous offspring, which would be more likely to 
succeed in the struggle for existence, and to perpetuate the 
characters of its parents. In this way we may suppose the pollen- 
grains of the Taxodieae, Cupresseae and Taxeae to have reached their 
present state. Ginkgo and the Cycads are at a lower level, and we 
may imagine that they represent the condition through which the 
Taxodieas, etc. passed before their male prothalli vanished entirely. 
But if this is the explanation of the loss of the male prothallus 
in the Taxodieae, Cupresseae and Taxeas, how is its survival in the 
Abietineae, and the distant genus Podocarpus, to be accounted for? 
I should like to suggest that in both these cases there is an actual 
correlation between the survival of the prothallus and the winged 
character of the pollen-grain. The pollen-grains of the Taxodieae, 
Cupresseae and Taxeae are not winged, while the specific gravity of 
those of the Abietineae and of Podocarpus is reduced by bladder¬ 
like swellings of the exine. The superior buoyancy thus produced 
would make any minute difference in weight of very little importance, 
and so natural selection would not have come into play to elimi¬ 
nate the vestigial sterile cells in the pollen-grain. One genus of the 
Abietineae, Tsuga, has wingless pollen-grains, but I have not been 
able to learn whether they contain any sterile cells. It certainly 
seems probable that we should find prothallial cells here, as in the 
other Abietineae, for in the case of a single wingless member of a 
family characterised by wings we must suppose that the absence of 
wings is not a primitive trait but a comparatively modern case of 
reduction, and hence it will not be surprising if there has not been 
time for the loss of the prothallial cells. 
In the temperate regions of the northern hemisphere Pinus is 
the dominant genus among the Conifers, while in the south its place 
is taken by Podocarpus. May we perhaps attribute the conspicuous. 
