1895. ^^^ ROLE OF SEX. 249 



clear-thinking biologist will, when it is thus put to him, assert that 

 because element A and element B produce a compound C, totally 

 unlike either A or B, that two conjugating individuals will do the 

 same. This is obviously not the result, and the biological facts must 

 therefore be taken and studied by themselves. These facts show that 

 when two individuals conjugate, their offspring tends to blend their 

 qualities, as when the cross between the negro and the white man 

 gives rise to offspring of an intermediate shade. Even where a quality 

 appears, like the colour of the eye, to be transmitted altogether or not 

 at all, Galton remarks that in these cases the tendency to blend is 

 never absolutely excluded.^ 



The plant or animal grows from the fertilised ovum, or from the 

 cutting, and in the case of some animals from an egg which has 

 not been fertilised at all. Here by asexual cell-division arise two 

 cells, which in the case of many animals vary slightly from each 

 other; their progeny continue this variation again and again, until 

 as a result such cells are reached as those of the brain, the blood, 

 the skin, the liver, and a hundred others. Embryology is a study 

 of deviating lines of variation from the fertilised ovum, from which, 

 by asexual reproduction, in the course of a few weeks, and in some 

 animals in a few days, we may obtain types totally dissimilar to 

 each other. This has always appeared to me to be the most striking 

 example of variation in existence, and we note its occurrence in 

 association with asexual reproduction. 



It is generally admitted that conjugation between members of 

 the same family fails to produce as vigorous a stock as crossing 

 between members of different families. Animals that interbreed are 

 proverbially delicate and wanting in physique, and this condition 

 reminds us of the infusoria of Maupas, which were kept from con- 

 jugating with infusoria of other families and which perpetuated 

 themselves asexually. Now, I do not see how this lack of vigour, 

 which is an outstanding feature of asexual reproduction and inbreed- 

 ing, can be explained on Weismann's theory. Although the environ- 

 mental conditions of life in a warren are the same to-day as they will 

 be in twenty years, yet the interbred progeny of a pair of rabbits 

 will seriously deteriorate and probably die out during that time. 



There must be something conservative in the act of crossing, 

 something which keeps up that vigour and vitality which is lost 

 without it. Now, this is easily explained if we consider the question 

 a little more closely, and it may assist us if we examine the family 

 histories of those of our acquaintances with which we are most 

 familiar. We know more as to the causes of lack of vigour in our 

 own species than in any other ; human pathology has advanced with 

 wonderful strides, and we have many facts to go upon. We learn 

 that there are very few families without some constitutional weak- 

 ness. Some are consumptive, others have a tendency to gout, etc. 



1 Op. cit. 



