3& PROBLEMS OF GENETICS 



animal like a zebra from a self-coloured type like a horse (or 

 of the self-coloured from the striped) as a process involving many 

 intergradational steps; but so far as the pattern is concerned, the 

 change may have been decided by a single event, just as the 

 multitudinous and ordered rippling of a beach may be created 

 or obliterated at one tide. 



This point is well illustrated by the tusk of an Indian elephant 

 which I lately found in a London sale-room. This tusk is by 

 some unknown cause, presumably a chronic inflammation, 

 thrown up into thirteen well-marked ridges which closely simulate 

 a series of segments (Fig. i). Whatever the cause the condition 

 shows how easily a normally unsegmented structure may be 

 converted into a series of repeated parts. 



The spread of segmentation through tissues normally unseg- 

 mented is very clearly exemplified in the skates' jaws shown in 

 Fig. 2. The right side of the upper figure shows the normal 

 arrangement in the species Rhinoptera jussieni, but the structure 

 on the left side is very different. The probable relations of the 

 several rows of teeth to the normal rows is indicated by the let- 

 tering, but it is evident that by the appearance of new planes 

 of division constituting separate centers of growth, the series has 

 been recast. The pattern of the left side is so definite that had 

 the variation affected the right side also, no systematist would 

 have hesitated to give the specimen a new specific name. The 

 other two drawings show similar variations of a less extensive 

 kind, the nature of which is explained by the lettering of the 

 rows of teeth. 



This power to divide is a fundamental attribute of life, and 

 of that power cell-division is a special example. In regard to 

 almost all the chief vital phenomena we can say with truth that 

 science has made some progress. If I mention respiration, meta- 

 bolism, digestion, each of these words calls to mind something 

 more than a bare statement that such acts are performed by an 

 animal or a plant. Each stands for volumes of successful ex- 

 periment and research. But the expression cell-division, the 

 fundamental act which typifies the rest, and on which they all 

 depend, remains a bare name. We can see with the microscope 



