17U 
In my former paper I selected certain animals which may be 
considered as lost to us, or are rapidly becoming so through the direct 
intervention of man, but to attempt to enumerate even the more 
sti iking sjiecies which have been lost under the conditions now to 
be considered would, I fear, be a hopeless task. 1 will, therefore, 
endeavour brielly to refer to .some of the agents which man luis, 
often unconsciously, employed, and the results which have arisen 
in con.sc(iuence. 
Ot the conditions under which what may be termed tlie natural 
death of a species occui-s we are profoundly ignonvnt; it is a 
process of which wo can take no cognizance. In ca.sting back over 
the geological record we sec how species after species, or even 
genus after genus, has come in, i-cachcd its maximum, and died 
out ; but as to tlie causes wliich led to the eflect we can form only 
a general idea ; all wo know is that at j)resent there are mysterious 
agencies working in nature, preventing the undue increase of 
certain species whose natural fecundity is unbounded, and, perhaps, 
fostering others, and that any interference with the natural 
cipiilibriuin thus set up will lead to disastrous I’csults. Mr. 
J )arwin, m considering the causes of extinction, Avrites as 
folIoAvs* : — 
“ 0 do not steadily bear in mind how profoundly ignorant we 
are of the conditions of existence of every animal ; nor do we 
always remember tliat some check is constantly 2)rcventing the too 
rai)ul increase of every organised being left in a state of nature. 
Ihc sup2>ly of food, on an average, remains constant; yet the 
tendenc) in every animal to increase by iwopagation is geometrical ; 
and its sur2)rising eflects have nowhere been morc astonishingly 
shoAvn than in the case of the European animals run Avild during 
the last fcAv centuries in America. Every animal in a state of 
nature regularly breeds ; yet in a species long-established, any 
Urcat increase in numbers is obviously impossible, and must be 
checked by some means. We are, nevertheless, seldom able Avith 
certainty to tell in any given species, at what period of life, or at 
what period of the year, or Avhether only at long intervals the check 
falls ; or, again, Avhat is the precise nature of the check. Hence, 
probably it is that Ave feel so little surprise at one, of tAvo .species 
* ‘Journal of Researches,’ &o., -iiuf etlit. p. 175. 
