PRESIDENTIAL ADDRESS. 47 
In spite of these striking exceptions, the axiom still holds 
good in a general way. Further, the presence of the vari- 
ous species still existent on the earth shows that these, in 
contradistinction to species now extinct, have been suffi- 
ciently prolific to maintain themselves up to the present 
in spite of the summation of all adverse conditions. For 
some species this has been difficult, and in the case of those 
verging on extinction the death rate has evidently exceeded 
the birth rate. In others, the maintenance has been easy. 
But all, rare species and common ones, are at any particu- 
lar period of time, and under normal conditions, relatively 
stable in numbers. If this be the case, then it is clear that 
the number of progeny, potential or actual, produced by the 
parent, or in the case of dicecious species the parents, dur- 
ing its, or their, lifetime, is a measure of the risks encoun- 
tered by the young in reaching maturity under average nor- 
mal conditions. In other words, each parent or pair of 
parents has the expectation, on the average, of leaving one 
or two descendants, according to the case, to perpetuate the 
species. Otherwise, of course, the number of individuals 
of the species would progressively increase or decrease. 
If a large number of progeny, either young animals or 
seeds, be produced, then it can be assumed that the chances 
against any one of these reaching maturity are great. If the 
number is small, then the risks are few. Many orchids 
produce annually vast numbers of seeds—evidently the like- 
lihood of any one of these finding a suitable habitat and 
maturing is very small, but the operation repeated from 
time to time during the life of the plant eventually, on 
an average, enables it to leave at least one descendant. The 
spores set free from the cap of one agaric (Psalliota cam- 
pestris—the common mushroom) have been estimated by 
Buller? at 1,800 million; from the fruit-body of another 
+ Buller, Researches on Fungi, 1909, pp. 82-85. 
