44 



Let us now pass on to consider the nature of the seed- 

 lings themselves. It was pointed out previously that 

 Darwin had shown the beneficial effect of cross-pollination 

 in the more vigorous development of the offspring. Another 

 important feature of seed reproduction as compared with 

 vegetative reproducton, particularly if the seeds are 

 the result of cross-fertilisation is the occurrence of a con- 

 siderable amount of variation. The more dissimilar the 

 parents the more varied are the offspring, and the greater 

 therefore the scope for the play of natural or artificial 

 selection. 



The term variation has been used for two very different 

 phenomena noticeable when examining a large number of 

 plants or animals of the same species or kind. It is well 

 known that the offspring of any two parents all have some 

 different individual characteristics, and a close observation 

 of a number of seedling plants will show us that though 

 they all have a general resemblance, we find that they 

 differ slightly one from another in size, in the shape and 

 texture of their leaves, and when they grow up in colour and 

 conformation of their flowers. Indeed, if we had carefully 

 examined the seeds from which they have grown we should 

 have found that the latter showed already a considerable 

 range of variation in shape and size, and possibly also in 

 colour. Such slight individual variations are always 

 found to fluctuate around a mean or average which we 

 may look upon as the general character of the species or 

 race. It is these slight individual variations which 

 Darwin regarded as so important in the evolution of new 

 forms, those least suited to the particular conditions of 

 life gradually dying out and leaving those which were fit 

 to survive. Natural selection operating in this way was 

 thought to have produced the innumerable forms which we 

 know as natural species. In the same way man, by making 

 a choice of the plants most suitable to his purposes, has 

 by artificial selection produced the strains and varieties 

 now cultivated. 



Recently experiments have been carried out in which 

 the seeds of certain plants have been carefully graded and 

 the largest sown separately with a view to ascertaining 

 whether by such constant selection the seeds could be in- 

 definitely increased in size. This was, however, not found 

 to be possible, for though the average size of the seeds was 

 at first considerably raised, a limit was reached beyond 



