Sottas—On the Origin of Freshwater Faunas. 117 
the throat of her young a sanguineous fluid which is probably similarly derived. 
Even when the diet is so digestible as fish, the young bird is not put to feed upon 
it always in the unprepared state, but is allowed to take it from the crop of the 
parent where it has probably undergone a partial digestion. 
The feeding of young pigeons with milk has been hitherto explained by the 
comparative helplessness of the young bird; but in that case what necessity is there 
for the special secretion of milk? One would have thought a grain diet could have 
been provided for it by the parent at a smaller expense to the parental resources. 
We have now to end this digression by pointing to one further consequence 
which follows upon the comparatively late assumption of an independent existence 
in the case of the higher animals. It is that in seclusion, where the play of 
incident forces is as little varied as possible, variations from the usual course of 
development are unlikely to occur, and consequently the variations which so con- 
stantly appear in a species are, as a rule, to be traced to the action of incident 
forces on the adult animal. A form that passes through a free larval development 
is hable to modification at any point of its development: one that does not is 
chiefly susceptible of modification after birth. Hence, as no crustacean appears 
upon the world at an earlier stage than a nauplius, so no crustacean can diverge 
from the phylum at a point lower down than that of the nauplius; while the 
lobster, being born fully formed, cannot develop into fresh varietal modifications 
except as these are moulded on the lobster type. This helps us to understand the 
tree-like form of zoological classifications. 
Returning now to the viviparous character of the development of Paludina, we 
find that it is not an isolated case amongst the Mollusca, for it occurs also among 
the Murexes; and further, that it is an illustrative case of a general tendency 
towards development in seclusion, the advantages of which are in most cases great, 
but particularly so in relation to a freshwater mode of existence. 
The views here advanced as to the origin of freshwater faunas may be briefly 
summarised as follows :— 
1. The conversion of comparatively shallow continental seas into freshwater 
lakes has taken place on a large scale several times in the history of the earth; 
and has been accompanied by the transformation of some of the marine into fresh- 
water species. 
2. At their inception these species may have been phanerogenous; if so, 
they have since nearly all become cryptogenous. This change has been accom- 
plished by the selective action of climate and river currents on varietal modifica- 
tions induced by change of climate (Hydra, Spongilla), or of food (? Paludina, 
pseudoparasitic glochidium), or otherwise. 
3. While apparently the only probable explanation of the origin of sedentary 
freshwater forms, the hypothesis will serve equally well to account for others 
which are locomotive; though in their case another explanation is possible, that 
of direct migration from the sea. 
