Chap. XXV. CORRELATED VARIABILITY. 319 



CHAPTEE XXV. 



LAWS OF VARIATION, continued — CORRELATED VARIABILITY, 



EXPLANATION OF TERM — CORRELATION AS CONNECTED WITH DEVELOPMENT — 

 MODIFICATIONS CORRELATED WITH THE INCREASED OR DECREASED SIZE OF 

 PARTS — CORRELATED VARIATION OF HOMOLOGOUS PARTS — FEATHERED FEET IN 

 BIRDS ASSUMING THE STRUCTURE OF THE WINGS — CORRELATION BETWEEN THE 

 HEAD AND THE EXTREMITIES — BETWEEN THE SKIN AND DERMAL APPENDAGES 

 — BETWEEN THE ORGANS OF SIGHT AND HEARING — CORRELATED MODIFICATIONS 

 IN THE ORGANS OF PLANTS — CORRELATED MONSTROSITIES — CORRELATION BE- 

 TWEEN THE SKULL AND EARS — SKULL AND CREST OF FEATHERS — SKULL AND 

 HORNS — CORRELATION OF GROWTH COMPLICATED BY THE ACCUMULATED EFFECTS 

 OF NATURAL SELECTION — COLOUR AS CORRELATED WITH CONSTITUTIONAL 

 PECULIARITIES. 



All the parts of the organisation are to a certain extent 

 connected or correlated together ; but the connexion may be so 

 slight that it hardly exists, as with compound animals or the 

 buds on the same tree. Even in the higher animals various 

 parts are not at all closely related ; for one part may be wholly 

 suppressed or rendered monstrous without any other part of 

 the body being affected. But in some cases, when one part 

 varies, certain other parts always, or nearly always, simulta- 

 neously vary; they are then subject to the law of correlated 

 variation. Formerly I used the somewhat vague expression of 

 correlation of growth, which may be applied to many large classes 

 of facts. Thus, all the parts of the body are admirably co- 

 ordinated for the peculiar habits of life of each organic being, 

 and they may be said, as the Duke of Argyll insists in his < Keign 

 of Law,' to be correlated for this purpose. Again, in large 

 groups of animals certain structures always co-exist; for in- 

 stance, a peculiar form of stomach with teeth of peculiar form, 

 and such structures may in one sense be said to be correlated. 

 Eut these cases have no necessary connexion with the law to 

 be discussed in the present chapter ; for we do not know that 



