QUANTITY PRODUCTION 273 



For example, our first cross, say in the case 

 of a prune, may be made between two varieties 

 that both show a fair quality of fruit. Careful 

 attention to the result will guide us in the 

 matter of the next experimental crossing. We 

 soon discover which qualities are prepotent, and 

 which tend to remain latent, and by selecting 

 only individuals that show a tendency to vary in 

 the desired direction, we introduce an element 

 of direction into the experiment. 



I am accustomed to speak of this as *'the mo- 

 mentum of variation." We do not always know 

 why a certain plant tends to vary in a given di- 

 rection, but we may observe the fact, and the 

 wise experimenter is always on the lookout for 

 this tendency and ready to avail himself of the 

 advantages it offers. Technical workers some- 

 times give the name "orthogenesis" to this tend- 

 ency to vary in a certain direction, mentioned 

 above as the plant's "momentum." 



Whatever aid we may gain in this way, 

 however, the manner of our advance is often 

 devious. 



In fact, it is very likely to be somewhat com- 

 parable to the progress of a sailing ship which 

 tacks this way and that, and which at times may 

 seem to be progressing in the wrong direction, 

 yet which in the end forges ahead. 



