Chap. XXV. CORRELATED VARIABILITY. 311 



CHAPTEK XXV. 



laws of variation, continued — correlated variability. 



EXPLANATION OF TERM CORRELATION — CONNECTED WITH DEVEOLrMENT — 

 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 BETWEEN 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 parts of the organisation are to a certain extent connected 

 together ; but the connection may be so slight that it hardly 

 exists, as with compound animals or the bnds 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, simultaneously 

 vary ; they are then subject to the law of correlated varia- 

 tion. The whole body is admirably co-ordinated for the pecu- 

 liar habits of life of each organic being, and may be said, as 

 the Duke of Argyll insists in his ' Reign of Law,' to be corre- 

 lated for this purpose. Again, in large groups of animals 

 certain structures always co-exist : for instance, a peculiar 

 form of stomach with teeth of peculiar form, and such 

 structures may in one sense be said to be correlated. But 

 these cases have no necessary connection with the law to be 

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

 the initial or primary variations of the several parts were in 

 any way related : slight modifications or individual differ- 

 ences may have been preserved, first in one and then in another 

 part, until the final and perfectly co-adapted structure was 



