30 VARIATION. 



have had this question answered can we expect a satisfactory- 

 answer to the second question : What causes, other than natu- 

 ral selection, can possibly reduce variability and render a 

 group of organisms stable. 



To Darwin, variability was a property of all living things, a 

 natural phenomenon, which could possibly be enhanced by 

 changes in the environment, by use or disuse of organs, but 

 withal one, fundamentally simple phenomenon. This is one 

 way of looking at variability, from the outside, and the term 

 variability comes to mean nothing more than a statement of 

 the fact that not all the individuals are identical. 



From the moment we look closely at the differences which 

 exist between the individuals of one group, we see that these 

 differences cannot be all of the same kind, and different only 

 in degree. If we observe a plot of wheat, we may observe that 

 the plants which grow in a wet spot are taller than those on 

 dry places of the same field. And, we perceive at once, that 

 this difference is fundamentally of another nature, than the 

 difference between an awned and an awnless plant which we 

 find growing in that field. We can now perform a simple exper- 

 iment, and sow four rows of wheat-kernels. One row from tall 

 plants of the wetter spot, one row from the lower plants of the 

 dryer portions, one row of seed from an awnless plant and 

 a row of seed from awned plants. We will then probably ob- 

 serve that the first two rows will grow up alike, the seeds from 

 taller plants will not give taller progeny than those from lower 

 ones. But at the same time we will see, that the seed from awn- 

 less plants gives awnless plants and that the grain from awned 

 plants reproduces awned off- spring. This shows, that the varia- 

 bility 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 seeds 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 



