48 _ J. B. CLELAND. 
(Coprinus comatus) at 5,240 million; and from a bracket 
fungus (Polyporus squamosus) at 11,000 million, and this 
was only one of ten brackets on the tree from the, same. 
mycelium. As, in these fungi, the one mycelium may give 
rise to several fruit-bodies during the season and maintain 
itself for years, evidently the likelihood of the germination 
and establishment of any one of these spores is almost in- 
finitesimal. Woman in a savage state had probably about. 
a dozen children, perhaps more, during her life time, but 
the risks attendant upon their upbringing must have reduc- 
ed the average of adults reaching maturity and procreating 
to little more than two. Amongst the British race, where 
celibacy is unfortunately far too common, it is considered 
that four births to a married couple are capable of main- 
taining the number of the population. 
From this it would appear that if we knew the average 
numbers of litters during the lifetimes of rats and mice, 
and the average number of young per litter, we should get 
an indication of the risks attendant on the young rats or 
mice before they are old enough to reproduce themselves. 
I have not got personal information of the average number 
of litters borne by our two common species of rats and the 
common house mouse during the individuals’ lifetimes. It 
is known that they reach maturity quickly, and that one 
pregnancy can be followed by another at intervals to be 
measured by weeks. I have, however, a considerable amount 
of evidence as to the number per litter, and some interest- 
ing points emerge from the consideration of this. My 
data show clearly that a large number of young must be 
produced during the average lifetime of these rodents, 
and that, therefore, the risks normally attendant on their 
lives, and especially on that of their young—due to starva- 
tion, man, beasts of prey, disease, accidents, and perhaps 
cannibalism—must be considerable; and conversely that, in 
