197 
begins to pulsate till it ceases to beat, the aeration 
of the blood is constant, and the function of respi- 
ration invariable ; carbonic acid is given off in the 
process, but the chemical change produced in the 
blood is unknown ; nor is there any reason to sup- 
pose the formation of any substance similar to 
sugar. In the production of a plant from a seed, 
some reservoir of nourishment is needed before 
the root can supply sap ; and this reservoir is the 
cotyledon in which it is stored up in an insoluble 
form, and protected, if necessary, during the win- 
ter, and rendered soluble by agents which are con- 
stantly present on the surface. The change of 
starch into sugar, connected with the absorption 
produce young ones, unless they are supplied with air, that is, 
unless the foetus can respire. I have found that the eggs of 
moths did not produce larvae when confined in pure carbonic 
acid ; and when they were exposed in common air, the oxygene 
partly disappeared, and carbonic acid was formed. The fish in 
the egg or spawn, gains its oxygene from the air dissolved in 
water ; and those fishqs that spawn in spring and summer in 
still water, such as the pike, carp, perch, and bream, deposit 
their eggs upon subaquatic vegetables, the leaves of which, in 
performing their healthy functions, supply oxygene to the 
water. The fish that spawn in winter, such as the salmon and 
trout, seek spots where there is a constant supply of fresh water, 
as near the sources of streams as possible, and in the most 
rapid currents, where all stagnation is prevented, and where 
the water is saturated with air, to which it has been exposed 
during its deposition from clouds. It is the instinct leading 
these fish to seek a supply of air for their eggs which carries 
them from seas or lakes into the mountain country ; which in- 
duces them to move against the stream, and to endeavour to 
overleap weirs, mill-dams, and cataracts. 
o 3 
