BI-PARENTAL REPRODUCTION 81 



selection must be very slow, so slow as to be inappreciable to 

 us. It follows also that the magnitude of any act of reversion 

 tends always to be greater than the magnitude of the correspond- 

 ing act of evolution. Thus many seminal generations must 

 have intervened between the crab and the cultivated apple, 

 but, in a single generation, the cultivated apple may, and 

 often does, revert far towards the crab. Therefore it is that, 

 in the absence of selection, species invariably undergo re- 

 gression. In this manner do parts long useless become mere 

 vestigial remains or disappear absolutely by reversion to that 

 ancestral condition in which they did not exist. Thus have 

 disappeared, for instance, the limbs of snakes, the eyes of 

 some cave-dwelling animals, and the many useless parts of 

 internal parasites. Thus have vanished utterly innumerable 

 useless parts in every species of plant and animal. 



140. Keturn now to bi-parental reproduction. An example 

 taken from disease will make its function clear to the reader. 

 Suppose any deadly malady, for instance malaria, is prevalent 

 in a country ; the men and women who survive and have off- 

 spring will in general be those who have varied favourably as 

 regards it. The children, therefore, will inherit the variation 

 undiminished by sexual reproduction and continued selection 

 will result in evolution. But we know, also, that people vary 

 in their powers of resistance to tuberculosis. Suppose, in the 

 country we are speaking of, tuberculosis is not prevalent. 

 Then people who vary favourably as regards it will not survive 

 in greater numbers than people who do not so vary. The two 

 types will survive equally and will mate together ; the older 

 type will tend to be prepotent, and this variation, useless 

 under the circumstances, will be planed away continually by 

 reversion to the ancestral type. The species will not become 

 burdened with it. If, however, tuberculosis be now intro- 

 duced, the situation is changed. Only those individuals will 

 survive that are resistant to the disease, and therefore evolu- 

 tion against it will proceed unchecked by sexual reproduction. 

 If, next, malaria be banished by improved sanitation, the 

 power, now useless, of resisting it will begin immediately to 

 undergo regression. The many individuals who revert to- 

 wards the more ancient and therefore the more prepotent type 

 in which it did not exist, will survive and have offspring. 

 What biologists call the " constitution of the race " will be 

 broken. It will become, to all appearance, much more variable. 



141. Indeed a little thought will render it evident that the 

 above considerations afford an explanation, at once probable 

 and in accord with the doctrine of Natural Selection, of the 



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