308 MERISTIC VARIATION. [part I. 



agreement with each other in colour-variation. The specimens 

 are all in the MacAndrew Collection in the Cambridge University 

 Museum, and I have as usual simply followed the labelling of the 

 specimens. 



C. arbust-uvi, Australia. 10 specimens, of which the plates in 6 are nearly uniform. 

 In one there is a white band in the centre of each plate ; in 2 the plates are irregularly 

 coloured; in one the plates 1 and 6 agree in being broadly marked with white. 



Chiton hennahi, Peru. 4 specimens. 3 are uniformly dark brown; but in the 

 other specimen there is a strong white mark on the centre of plates 2 — 7, and a faint 

 one on plates 1 and 8. 



C. elegans, Chili. 2 specimens. In one, complicated markings are repeated on 

 each plate nearly uniformly; in the other specimen a much simpler pattern recurs on 

 each segment. 



On the other hand, C. pellis-serpentis, New Zealand, 8 specimens: great di- 

 versity of markings and no uniformity among plates in 4 specimens, but in one 

 specimen plates 2 — 5 were black and the rest light-coloured. Similar want of uni- 

 formity among the plates in 2 specimens of C. incanus, New Zealand. 



The evidence of agreement between segments 2, 4 and 7 in the following cases is 

 very striking. 



C. (Tonicia) marmoreus, "Hebrides, &c." 18 specimens, all of a light brown 

 colour marked with dark red. 



In 4 specimens the plates are uniformly marked or nearly so. 



In 6 specimens plates 2, 4 and 7 are much darker than the others, being for the 

 most part of a uniform dark red. 



In 5 specimens plates 2, 4, 7 and 8 are darker than the rest. 



In 1 specimen plates 2, 4, 5 and 7 are darker than the rest. 



In 2 specimens the central parts of most of the plates have dark markings, but 

 no segment is specially distinguished. 



Of 18 specimens therefore 12 have plates 2, 4 and 7 darker than the rest. 



Among 3 specimens of the same species from Gr. Manan (N. America) 2 are nearly 

 uniform throughout, but in one plates 2, 4, 7 and 8 are much darker than the rest. 



G. (Tonicia) lineatus, 2 specimens. In one the markings on all the plates are 

 nearly similar, and the white wavy streaks characterizing the species are almost 

 similarly distributed on the sides of all the plates. In the other specimen these 

 lines are absent on the plates 2, 4 and 7, which are much darker than the rest ; but 

 the lines, though less extensive than in the first specimen, are present on plates 

 1, 3, 5, 6 and 8. 





The preceding evidence may suffice to indicate the nature of 

 this important question of the degree to which the colour-variations j 

 of parts repeated in Linear Series may be similar and simultaneous, j 

 a question which, as must be evident, is of the highest consequence 

 in estimating the magnitude of the steps by which Evolution 

 may proceed. To the consideration of this matter it will be 

 necessary to return when the evidence of Substantive Variation : 

 is considered. 



Meanwhile it will not be forgotten that though we have only 

 spoken of this question in reference to colour and to Linear Series, j 

 the same question arises also with regard to other variations and in j 

 reference to all parts which are in any way repeated and resemble 

 each other, whether such repetition is strictly serial or not. In a 

 survey of any group of animals cases will be seen in which organs 

 in one region are repetitions of organs in another region though 





