VARIATION. 33 



kind and hang them up to dry in the sun, the water in the seeds 

 evaporates, and as the envelop of the seed becomes too wide 

 for the shrinking contents, it wrinkles, so that finally the seeds 

 have a shrunken and somewhat glassy appearance like raisins. 



If now we compare two ears of plants of different species, 

 which we leave to ripen on the plants, we see that the ears do 

 not behave in the same way. The seeds of the sugar-corn behave 

 almost exactly in the way in which those in the cut ear did. 

 The sugar concentrates, and as the water evaporates and 

 the seed dries, it shrinks and becomes wrinkly and glassy. In 

 the other plants, in the ripening seeds, the sugari s transform- 

 ed into starch, and as a result of this process the seeds become 

 hard, and on drying they remain so, and retain their shape. 

 Therefore, we see that this difference between the hard starch- 

 corn and the wrinkly sugar-corn, must be due to a develop- 

 mental factor which at the time the seeds are ripening, is indis- 

 pensable for a transformation of sugar into starch. It is not 

 necessary to assume that this factor is in itself responsible for 

 this transformation of sugar into starch, just as little as we 

 need to assume, that calcium is alone responsible for the form- 

 ation of the second membrane in the egg of the sea-urchin. 



We must state it thus : for the formation of this second mem- 

 brane in the sea urchin, one of the indispensable factors is cal- 

 cium, and for the transformation of sugar into starch in ripe- 

 ning corn-seeds, a certain heritable developmental factor is 

 indispensable. We can never say that calcium causes the second 

 membrane, that this membrane is determined by calcium. It 

 may be possible to find another factor which is just as indis- 

 pensable as calcium, for instance oxygen, and in the case of the 

 corn we may not say that this one factor determines starch or 

 starchiness. It is simply one thing which is necessary for the 

 formation of starch. 



In the example of the sea-urchin embryo, we are dealing with 

 a developmetal factor of which the nature is known to us, and 

 we know, furthermore, that it influences the development from 

 without. In the corn-example we cannot lay our hands on the 



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