334 Evolution and Adaptation 
oO 
Since we are entirely in the dark as to how much time has 
been required for the formation of phyla, so also are we 
ignorant as to how long it may have taken for each step in 
advance. We may err equally in ascribing too much and 
too little time to the process. It is, moreover, not necessary 
that for every step the same amount of time should have 
been required. On the contrary, the probability is that 
recognizable changes may at times follow each other rapidly, 
and then for a time come to a standstill, —just as in the 
development of the individual there are periods of more rapid 
and others of less rapid change. 
A more difficult problem than that relating to the sort of 
changes the external influences bring about in the organism, 
is the question as to how they effect the organism, or how 
they act on it mechanically. This, as is well known, was 
answered by Darwin, who regards all organization as a prob- 
lem of adaptation: only those chance variations surviving 
which are capable of existence, the others being destroyed. 
On this theory external influences have only a negative or a 
passive action, namely, in setting aside the unadapted indi- 
viduals. Ndageli, on the other hand, looks upon some kinds 
of external conditions as directly giving rise to the adaptive 
characters of the organism. This is accomplished, he sup- 
poses, in the following ways: two kinds of influence are 
recognized; the direct action, which, as in inorganic nature, 
comes to an end when the external influences come to an 
end, as when cold .diminishes the chemical actions in the 
plant; and ¢he cudirect action, generally known as a stimulus, 
which starts a series of molecular motions, invisible to us, 
but which we recognize only in their effects. Very often 
the stimulus starts only a reflex action, usually at the place 
of application. 
A stimulus acting for but a short time produces no last- 
ing effect on the idioplasm. A person stung by a wasp 
suffers no permanent effect from the injury. But if a stim- 
