COLOUR AND PATTERN IN ANIMALS 65 



crease along the middle, and then unfolding it, write a name in ink 

 with a thick pen along one side of the crease, then quickly fold it 

 over and press it down before the ink has dried, we shall find we 

 ( have made another kind of pattern (Fig. 21), this time not 

 ^radially symmetrical round a central point, but bilaterally 

 symmetrical on the two sides of the crease, and more complicated 

 in detail because of the different thicknesses of the ink we left to be 

 doubled and the unconsciously different 

 pressures we gave when folding over the 

 paper on the wet ink. 



The growth of every body takes place by 

 the multiplication of the little units we 

 know as cells, or of higher units composed 

 of masses of cells. Sometimes the multi- 

 plication takes place radially and regularly, 

 or radially and irregularly, sometimes in a 

 bilateral plane, and this again regularly or 

 irregularly. And so all the tissues of the 

 body, microscopic or visible to the naked 

 eye, are patterned. In the simpler forms of 

 life and the simpler, most mechanical parts 

 of the body, the patterns are simple and 

 regular, to whatever system they may belong. 

 In the higher tissues and higher organisms 

 the primitive numerical symmetries of re- FIG. 21. Bilateral Pattern 

 petition are disguised by ordered irregu- han^writin 7 ^theStual 

 larities in growth, now one part, now another words written a were 

 part being retarded or accelerated, and by ' R y al institution." 

 the interference of the growth-forces of one set of organs with the 

 ,growth-forces of another. If a drop of some oily pigment be placed 

 on water in a bowl it will spread out slowly in a ring-shaped pattern ; 

 if other drops be placed near it, as they spread they will interfere with 

 and distort the patterns already formed. If the water be made to 

 move slowly by stroking the surface with a brush or by blowing on it, 

 the systems of rings will spread out into irregular curved streaks, 

 forming the well-known watered or moire effects used in textiles, 

 and sometimes seen on the paper lining the covers of books. Similar 

 patterns are very common in animal tissues, due to the growth- 

 forces being more intense in one direction than in another. Thus 

 in a multitude of ways patterns are formed in the tissues of animals, 

 as the inevitable consequence of structure and mode of growth, and 



c.A. E 



