No. I.] ORGANIC VARIATION. 303 
in other words, how great a change of environment the organism 
can withstand without serious injury. Now from a number of 
carefully made experiments, as noticeably the recent observa- 
tions of Davenport and Castle,! we find that an organism which 
would be killed by a sudden change of temperature of 10° C., 
may become acclimated to that amount of increase in tempera- 
ture if the change is made gradually. This fact proves that an 
organism can withstand only a certain maximum amount of 
sudden change of environment, while if the change be greater 
than this maximum amount, death ensues. Still another fact is 
important in this connection : lowly organized forms are, as a 
rule, more wéderstandsfahig than highly organized forms; it 
suffices, as an example, to call to mind the great changes of tem- 
perature which are not injurious to certain disease germs and 
swarm-spores, but which would cause the sudden death of a worm 
or mammal. From these facts we may conclude: (1) that as a 
given organism can withstand only a certain maximum amount 
of change in the environment, and since the amount of variation 
stands in a direct ratio to the amount of change in the environ- 
ment, therefore the organism can produce only a certain 
maximum amount of variation; and (2), since lowly organized 
forms can withstand greater changes of environment, as a rule, 
than can more highly organized forms, that the former can, as 
a rule, produce a greater amount of variation than the latter 
can. And these facts coincide perfectly with the general physi- 
ological law that the more differentiated the organs become 
structurally, the more intimate and complex becomes their 
correlation ; for more highly differentiated organisms, with a 
more complex correlation of their organs, are unable to produce 
variations to the same amount as can more lowly organized forms 
which have a less intimate correlation of their organs. Further, 
the correlation of the organs in lowly organized forms, being 
less complex, can be restored sooner after a change of environ- 
ment than the correlation can be restored in more highly dif- 
ferentiated forms after the same amount of change. Thus we 
find that the amount of variation depends upon the degree 
1“On the Acclimatization of Organisms to High Temperatures,” Arch. f. 
Entwicklungsmech. d. Organismen, Bd. II, 1895. 
