8 Experimental Zoology 
but only because we have found that death always takes place 
under ordinary conditions. Suppose, however, we change the 
conditions; might we not hope to prolong the duration of life? 
Improbable as this may seem, there are already experiments 
afoot that indicate that, however difficult, the problems may 
not be insoluble. 
Why, on an average, in most animals, are equal numbers of 
two forms born —male and female? Is there an internal 
mechanism? If so, what regulates it? Do external or inter- 
nal conditions determine that one egg becomes male, another 
female? Even if an internal mechanism exists, it might be 
affected by external conditions, and in any case the cause of 
the production of the two types must be determined. 
Observation has established that the evolution of animals and 
plants has, in all probability, taken place. But what factors are 
involved in the process are unknown. Only in the last few 
years by means of an experimental study of the subject has 
decided advance been made. 
It is sometimes stated that nature has already carried out 
innumerable and wonderful experiments, and that we can 
never hope to excel her in this power. Is it not better, therefore, 
to examine patiently and reverently what she has done, and in 
this way learn how her processes have been carried out? Let 
us not be blinded by rhetorical questions of this kind. No 
doubt nature has carried out prodigious experiments; but we 
can never be certain that we know how she has obtained her re- 
sults until we can repeat the process ourselves. What would 
the chemist or the physicist say if he were told that nature has 
already carried out experiments on a much greater scale than 
he can hope to accomplish, and that he should drop his ex- 
perimental methods and. study his physics in a thunderstorm 
and his chemistry in a volcanic eruption ! 
I have brought up this point because it illustrates one side 
of the experimental method that is sometimes overlooked. Al- 
most all of the phenomena with which the biologist has to deal 
are so complex that he cannot determine what part each factor 
