chap, xii.] SIMULTANEOUS COLOUR- VARIATION. 303 



Simultaneity of Colour-variation in Parts repeated in 



Linear Series. 



Reference was made (Introduction, Section V.) to that relation 

 subsisting between the several members of a linear series of 

 segments or other repeated parts, by virtue of which they may 

 resemble each other in respect of colour or pattern of colours 

 From the fact that the several members do in such cases often 

 bear the same colours or patterns it is clear that they must at 

 some time or other have undergone similar Variation. In order 

 to measure the possible rapidity of the process of evolution by 

 which such parts may have reached their present condition it is 

 important to ascertain the extent to which their several variations 

 may be simultaneous. 



Variations in colour are of course Substantive variations and a 

 full consideration of their nature cannot be taken here. For the 

 present we are only concerned with the consequences of the fact 

 that the parts are repeated in series. As was pointed out in the 

 Introduction the problem of the resemblance between the colours 

 of such segments is only a special case of the same problem of 

 Symmetry which is again presented in bilateral or other Repetition. 



Simultaneous colour-variation taking place abruptly in a large 

 number of organs, such as hairs, feathers, &c. is a very common 

 occurrence, and the part that repetition of structures plays in 

 producing the total effect is apt to be overlooked. In comparing 

 two varieties of some whole-coloured animal, a bay horse with a 

 chestnut for example, it must be remembered that the difference 

 is really made up of a simultaneous variation in the pigment of each 

 particular hair. Similarly if a caterpillar normally green appears 

 in a uniformly brown variety we may conceive the total change as 

 brought about by variation occurring simultaneously in the skin 

 of the several segments, or in some smaller units. But whatever 

 unit be taken, whether segment, or hairs, or cells, that all or any 

 particular groups of such units should vary together and in the 

 same direction is not a matter of necessity. That such simul- 

 taneity is not universal and that segments may vary independently 

 of each other is a matter of common observation, and indeed is 

 sufficiently proved by the occurrence of differentiation between 

 segments. Nevertheless the evidence goes to shew that between 

 parts repeated in series there may be a relationship of the kind 

 spoken of, though its causes, nature and limitations are unknown. 

 In the case of actual segmentation this relationship may appear 

 either in the simultaneous variation of the colour-patterns of the 

 segments, or of some one colour or patch borne by each, or by the 

 appearance of some unusual mark or patch on several of them at 

 once. 



In some cases it happens that certain of the segments may 

 vary together, the rest remaining unchanged, and, as seen in 



