373 
Darwin observed that sterility may be occasioned by close 
interbreeding,* or due to crosses between two distinct species, 
“for it is scarcely possible that two organisms should be com- 
pounded into one without some disturbance occurring in the 
development or periodical action, or mutual relations of the 
different parts and organs one to another, or to the conditions 
of life. When hybrids are able to breed inter se they transmit 
to their offspring, from generation to generation, the same 
compounded organization, and hence we need not be surprised 
that their sterility, though in a degree variable, rarely 
diminishes.” (Origin of Species, page 266.) 
The sterility of crosses among pure species wherein the 
reproductive organs are perfect, often, not always, depends on 
the early death of the embryo: while the sterility of mules 
possessing imperfect generative conditions is allied to that 
condition in pure species caused by the natural conditions of 
life having been changed. 
* Although among mammals interbreeding from species of close con- 
sanguinity has been found deleterious to the offspring, experiments are still 
desirable as to whether similar results occur as we descend the scale of 
animal life? It is evident that the offspring of young parents or of hybrids 
are not vigorous among fishes, also that such as are irregularly hatched, owing 
to abnormal conditions of the water, or when the young are underfed or 
overcrowded, no good fish will be reared, any more than they will if there is 
insufficient food in the waters into which they are turned. But it has not 
been shown that eggs from small races, when properly treated, will not give 
results possibly nearly equal to those from large races, or that the offspring, 
if well cared for, will not attain a large size. 
AA 
