DARWINISM AND EVOLUTION. 23 



practised in South America, in Greenland, Kamskatka, West 

 Yunnan, Borneo, in the North of Spain, in Corsica, and in the 

 South of France. Mr. Octavius March wrote to Mr. Darwin in 

 1875, asking him whether, in connection with the fact that male 

 mammals possess mammae, he thought it possible that the couvade 

 had its origin in an early habit of the male sex to take part in 

 the nourishment of the offspring. Mr. Darwin replied as follows. 

 "The couvade has always seemed to me the most extraordinary of 

 " all the odd customs followed by man. Your idea is new to me, 

 " and very ingenious ; but I do not think that it can be admitted, as 

 " the period when male mammals suckled their young must have 

 " been so immensely remote, seeing that rudimentary mammae 

 " are common to all orders, including the marsupials." 



Neither correspondent seems to have known that in some 

 African villages, as attested by Livingstone and other travellers, 

 elderly men with well-developed mammae are employed as wet 

 nurses- in suckling the young. 



The fact must here be noticed that an animal's form becomes 

 modified in consequence of a long continued alteration of its 

 habits. 



We might feel sure that, if animals had at any former period 

 entirely changed their mode of life, we should find in proportion 

 to the length of time that had elapsed since such change began, 

 some organs would have become modified to suit the altered 

 conditions, and other organs not needed would have become 

 rudimentary. At various times, creatures that used to live on the 

 land have been driven, in search of food, to betake themselves 

 to the water. Two instances can be given of this transfer, in its 

 incipient stage. "In the case of the water-ouzel the acutest 

 "observer," says Mr. Darwin, "by examining its dead body would 

 " never have suspected its subaquatic habits ; yet this bird which 

 " is allied to the thrush family, subsists by diving — using its wings 

 " under water, and grasping stones with its feet. All the Hymen- 

 "opterous insects are terrestrial, except the genus Proctotrupes, 

 "which often enters the water and dives about by the use not of 

 "its legs, but of its wings, and remains as long as four hours 

 " beneath the surface ; yet it exhibits so far no modification of 

 "structure in accordance with its abnormal habits." Some 

 internal modification, however must necessarily have taken place, 

 especially as regards its organs of respiration. 



An instance can be given of an older stage of this transfer, so 

 that a change of structure visibly exists. The carnivorous mollusc 

 Ianthina, is thought to have taken to the sea in recent times 

 because it has never been found in a fossil state. Its shell closely 

 resembles that of a common land snail. Its foot secretes a remark- 

 able structure, considered to be the analogue or corresponding 

 organ of the operculum (lid). This consists of a large raft 



