28 
'common tape-worm lays hundreds of millions of eggs in a 
very short time, yet it is comparatively rare. The number of 
children horn to each pair of human beings during their life- 
time of from fifty to eighty years, can be counted on the 
fingers, yet man is the most abundant of the larger mammals, 
and human population increases quite rapidly under favora- 
ble circumstances. This comparison shows plainly that the 
abundance of a species is determined, mainly, by the exter- 
nal conditions to which it is exposed, and that the num- 
ber of individuals which are born has very little to do with 
it. In the case of the oyster, the adult is well protected 
against enemies by the shell, and as its food is abundant, and 
is brought to it by the water, it is tolerably sure of a long 
life after it has reached its adult form, but the life of the 
young is very precarious; that of the young American oyster 
peculiarly so, since it is exposed to all kinds of enemies and 
accidents, at a time when it is most helpless. The protection 
of the young European oyster by the parent shell at this time 
would seem to more than balance the greater number of eggs 
laid by the American. 
The most critical time in the life of the American oyster is 
undoubtedly the time when the egg is discharged into the 
water to be fertilized, for the chance that each egg which 
floats out into the ocean to shift for itself will immediately 
meet with a male cell is very slight, and it is essential that 
the egg should be fertilized very quickly, for the unfertilized 
egg is destroyed by the sea water in a very short time. The 
next period of great danger is the short time during which 
the embryos swarm to the surface of the water. They are so 
perfectly defenceless, and so crowded together close to the sur- 
face, that a small fish, swimming along with open mouth, 
might easily swallow in a few mouthfulls a number equal to 
the human population of Baltimore. They are also exposed 
to sudden changes of temperature, and as my experiments 
have shown that a sudden fall in temperature is fatal to them 
at this time, the number which are destroyed by cold rains 
and winds must be very great indeed. 
As soon as they are safely past this stage, and scatter and 
