chap, xil] SIMULTANEOUS VARIATION OF SERIES. 309 



not necessarily in serial homology with them in any sense in 

 which the term is commonly used. Many such cases were spoken 

 •of by Darwin in the chapter on " Correlated Variability 1 " and are 

 now famous. The simultaneous colour- variations of the mane and 

 tail of horses 2 , the correspondence between the large quills of the 

 wings and those of the tail of pigeons 3 and other birds are 

 among the most familiar of such cases. 



When with such facts in mind we turn to some species which 

 differs from an ally in the presence of some characteristic develop- 

 ment or condition common to a number of its parts, in making 

 any estimate of the steps by which it may have been evolved it 

 must be remembered that it is at least possible that the common 

 feature characterizing these several parts may have been assumed 

 by all simultaneously. To take a single instance of this kind, the 

 species of the genus Hippocampus, the Sea-horses, have the 

 shields produced into more or less prominent tubercles or spines. 

 The back of the head is also drawn out into a prominent knob. 

 In^ an allied genus from Australia, Phyllopteryx, many of these 

 spines are provided with ragged looking tags of coloured skin, 

 like the seaweed which the fishes frequent 4 , giving the animal a 

 most fantastic appearance and no doubt contributing greatly to 

 its concealment [probably from its prey]. If in this case it were 

 nece>sary to suppose that the variations by which this form has 

 departejj from the ordinary Hippocampi had occurred separately, 

 and that -each spine had separately developed its tag of skin, the 

 number of variations and selections to be postulated would be 

 enormous ; but probably no such supposition is needed. We are, 

 as I think, entitled to expect that if we had before us the line of 

 ancestors of Phyllopteryx, we should see that many and perhaps 

 all of the spines which are thus modified in different parts of the 

 body had simultaneously broken out, as we may say, into tags of 

 skin, just as the feathers of the Moor-hen (Gallinula chloropus) 5 

 may collectively take on the " hairy " form, or as, to take the case 



1 Animals and Plants under Domestication, ed. 1885, n. chap. xxv. 



2 As D,*$*vin mentions, simultaneity in the variations of the hair may be mani- 

 fested in size and texture as well as in colour. A bay horse was lately exhibited at 

 the Westminster Aquarium standing 16£ hands, having the hair of both mane and 

 tail of prodigious length. The longest hairs of the mane measured 14 ft. and those 

 of the tail 13 ft. It did not appear that the hair of the fetlocks or bo ly was unusual 

 in character, but these were kept closely clipped and nothing could be affirmed on 

 this point. 



3 By the courtesy of Professor L. Vaillant I was enabled to examine a number 

 of specimens of the singular breeds uf Gold-fish from China in the Paris Museum of 

 Natural History. Some of these are characterized by the great length both of the 

 appendicular fins and of the caudal fin also. Measurement shewed that there was 

 a substantial correspondence between the lengths of these parts, those with long 

 appendicular fins having also very long tails. The correlation between these parts 

 is not however universal in Gold-fishes, and in many of the ordinary "Telescope" 

 Gold-fish the tail may be longer than that of a common Gold-fish of the same size, 

 though the length of the appendicular fins be not exceptional (v. infra). 



4 Gunther, Study of Fishes, 1880, p. 682, fig. 309. 



5 See Introduction, p. 55. 



