Cuai\ XXVI. SUMMARY. 347 



curly hair have hoofs like those of a mule ; the wool and the 

 horns of sheep often vary together ; hairless dogs are deficient 

 in their teeth ; men with redundant hair have abnormal teeth. 

 either by deficiency or excess. Birds with long wing-feaihers 

 usually have long tail-feathers. When long feathers grow 

 from the outside of the legs and toes of pigeons, the two 

 outer toes are connected by membrane; for the whole leg 

 tends to assume the structure of the wing. There is a mani- 

 fest relation between a crest of feathers on the head and a 

 marvellous amount of change in the skull of various fowls ; 

 and in a lesser degree, between the greatly elongated, lopping 

 ears of rabbits and the structure of their skulls. With plants, 

 the leaves, various parts of the flower, and the fruit, often 

 vary together to a correlated manner. 



In some cases we find correlation without being able even 

 to conjecture what is the nature of the connection, as with 

 various monstrosities and diseases. This is likewise the case 

 with the colour of the adult pigeon, in connection with the 

 presence of down on the young bird. Numerous curious 

 instances have been given of peculiarities of constitution, 

 in correlation with colour, as shown by the immunity of 

 individuals of one colour from certain diseases, from the 

 attacks of parasites and from the action of certain vegetable 

 poisons. 



Correlation is an important subject ; for with species, and 

 in a lesser degree with domestic races, we continually find 

 that certain parts have been greatly modified to serve sortie 

 useful purpose; but we almost invariably find that other 

 parts have likewise been more or less modified, without our 

 being able to discover any advantage in the change. No 

 doubt great caution is necessary with respect to this latter 

 point, for it is difficult to overrate our ignorance on the 

 use of various parts of the organisation ; but from what we 

 have seen, we may believe that many modifications are of no 

 direct service, having arisen in correlation with other and 

 useful changes. 



Homologous parts during their early development often 

 become fused together. Multiple and homologous organs 

 are especially liable to vary in number and probably ic 



