166 Dr. W. Marshall on the 
which precisely in these species escape very easily from the 
dead Spongilla, with which they are not in the least united. 
The wind will take them up, scatter them here and there over 
the great plains of Africa &c., and deposit them in dried-up 
watercourses, in which they will be found by the vivifying 
element at the commencement of the rainy season. This is 
not contradicted by the fact that others come to rest in peren- 
nial fresh waters and develop there; many will be carried 
far away to islands and from land to land, many will get into 
the sea and never fulfil their destiny ; but if, out of their great 
number, the greater because they are so small, only a very 
small percentage arrives at development, the preservation of 
the species is thereby abundantly assured. 
What a means of transport for organic substances the wind 
is we may learn from the works of the honoured master 
Ehrenberg ; out of the 1200 figures which he gives of organ- 
isms obtained from samples of dust carried by the trade- 
winds, no fewer than 285, or, in round numbers, 24 per cent., 
are evidently remains of sponges ; and of these 46, or, in round 
numbers, 4 per cent. of the whole, or nearly 16 per cent. of 
the sponge-fragments, are fractured or entire amphidisci of 
various species of Spongille. 
By far the greater part of the organic remains figured by 
Ehrenberg are derived from fresh water: we find among them 
Diatomee still with green contents rich in chlorophyll; the 
marine objects, Polythalamia, sponge-spicules (some of which 
are of deep-water forms, such as Geodiew and Hexactinellide), 
are probably not, as Ehrenberg supposed, recent, but originate 
from the Tertiary deposits of North-west Africa (Oran), which 
contain such an abundance of fossils. ‘That among this dust, 
which therefore originates from Africa, and not, as Ehren- 
berg supposed, from South America, to be thrown down in 
Europe, there are no such large specimens as the gemmules 
of Spongilla nitens for example, proves scarcely any thing. 
The further the particles of dust are carried from the regions 
in which they were taken up, the finer will they be, and vice 
versa. According to the weight of the objects transported a 
sort of sifting of the atmosphere will gradually take place! 
I have experimented, certainly with the roughest apparatus, 
in the following manner :—a number of gemmules from speci- 
mens of Spongilla lacustris and S. nitens {from the White 
Nile, in the Leipzig Museum), which had already been pre- 
served dry for many years, were further dried at a moderate 
heat under the same conditions for eight days; then fifty of 
each kind were taken and mixed together, and then placed in 
a little heap at one end of a perfectly flat newly polished 
