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THE GRAMMAR OF SCIENCE 



both sides of the backbone/ and this forms a simple 

 " character " in the leaf to which we can give a numerical 

 value. We soon find that two leaves are not necessarily- 

 alike in this character, the veins vary in number from ten 

 to twenty-two. We are not concerned as to whether this 

 variation is due to environment or to " inherent growth- 

 force " ; we simply say the character varies. 



We now collect twenty to thirty leaves from different 

 parts of one beech tree, and twenty to thirty leaves from a 

 second. We count the veins on one series of leaves and 

 again on the other. The two series distinctly differ ; 

 here are two actual examples : — 



Now, so far as this character is concerned, we might have 

 interchanged certain of the individual leaves, but we 

 could not have interchanged the two series. They are 

 individual to the two trees. Now in what does this 

 individuality consist ? Clearly there are most leaves in 

 one tree with eighteen ribs, and most in the other with 

 fifteen ribs. A numerical value for which such a frequency 

 is greatest is termed a modal value or mode. If we took 

 the modes for a great variety of characters, we should 

 have the type. The type of one beech tree differs from 

 another, and it is the type which determines the indi- 

 vidual. Look at the same sort of idea in the wild 

 poppy. The reader must be familiar with the dark 

 rays on the top of the seed capsule of the poppy, 

 the so-called stigmatic bands. These are determined in 

 the bud, and their number has nothing to do with 

 fertilisation. They can be easily counted, and form a 

 simple character of the poppy. Here are the results for 

 two wild poppies : 



1 Of course near the tip of the leaf the determination of the number of 

 veins becomes more difficult, but with careful attention to light and shade, 

 the existence or not of a small and insignificant vein maybe generally ascertained. 



