36 PROBLEMS OF GENETICS 



the analysis applicable to the mechanical patterns are applicable 

 to the zebra stripes also. Patterns mechanically produced are 

 of many and very diverse kinds. One of the most familiar 

 examples, and one presenting some especially striking analogies 

 to organic patterns, is that provided by the ripples of a mackerel 

 sky, or those made in a flat sandy beach by the wind or the ebbing 

 tide. With a little search we can find among the ripple-marks, and 

 in other patterns produced by simple physical means, the closest 

 parallels to all the phenomena of striping as we see them in our 

 animals. The forking of the stripes, the differentiation of two 

 "faces," the deflections round the limbs and so forth, which in the 

 body we know to be phenomena of division, are common both to 

 the mechanical and the animal patterns. We cannot tell what in 

 the zebra corresponds to the wind or the flow of the current, but 

 we can perceive that in the distribution of the pigments, that is 

 to say, of the chromogen-substances or of the ferments which 

 act upon them, a rhythmical disturbance has been set up which 

 has produced the pattern we see; and I think we are entitled to 

 the inference that in the formation of patterns in animals and 

 plants mechanical forces are operating which ought to be, and 

 will prove to be, capable of mathematical analysis. The com- 

 parison between the striping of a living organism and the sand- 

 ripples will serve us yet a little farther, for a pattern may either 

 be formed by actual cell-divisions, and the distribution of dif- 

 ferentiation coincidently determined, or — as visibly in the pig- 

 mentation of many animal and plant tissues — the pattern may 

 be laid down and the pigment (for example) distributed through 

 a tissue across or independently of the cell-divisions of the tissue. 

 Our tissues therefore are like a beach composed of sands of 

 different kinds, and different kinds of sands may show distinct 

 and interpenetrating ripples. When the essential analogy be- 

 tween these various classes of phenomena is perceived, no one 

 will be astonished at, or reluctant to admit, the reality of dis- 

 continuity in Variation, and if we are as far as ever from knowing 

 the actual causation of pattern we ought not to feel surprised that 

 it may arise suddenly or be suddenly modified in descent. Biol- 

 ogists have felt it easier to conceive the evolution of a striped 



