By Arend L. Hagedoorn Ph. D, A. C. Hagedoorn-Vorstheuvel La Brand (auth.)

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The mechanism of the response may be such an one, that at a critical point in the relation between the action of one factor and the others, the organism begins to react differently. Some phenomena, certain processes may by their very nature prohibit continuous variation. For instance, meristic phenomena. We do not know many definite facts about the causes for the position of leaves on a stem, but we can see how the very nature of the obscure mechanism makes it impossible for a stem to bear its leaves in any way intermediate between two definite ones.

This shows, that the variability is different in the two instances. We see that in the last case, the difference between the awnless and the awned plants corresponds to a difference between the ~s these plants produce, but we see also that a difference between two plants does not necessarily imply a similar difference between the seed produced. In studying variation and its causes, it is therefore necessary VARIATION. 31 to ask, what is the relation between the quality of a seed and the qualities ofthe plants?

VARIATION. DARWIN in his theory of evolution started from the observation that all organisms are seen to vary in every character, in every proportion, every function which is studied. Taking for granted this variability, Darwin reasoned, that such small differences as always exist between members of a single group, must make these individuals better or less-fitted for the conditions under which they have to live. Andasitis manifestly impossible, for all the descendants of all the individuals to find a place on earth, a portion only can survice in each generation.

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