2 8 READINGS IN BIOLOGICAL SCIENCE 



The reproductive system offers various rudimentary structures; but these 

 differ in one important respect from the foregoing cases. Here we are 

 not concerned with the vestige of a part which does not belong to the 

 species in an efficient state, but with a part efficient in the one sex, and 

 represented in the other by a mere rudiment. Nevertheless, the occurrence 

 of such rudiments is as difficult to explain, on the belief of the separate 

 creation of each species, as in the foregoing cases. Hereafter I shall have 

 to recur to these rudiments, and shall shew that their presence generally 

 depends merely on inheritance, that is, on parts acquired by one sex having 

 been partially transmitted to the other. I will in this place only give some 

 instances of such rudiments. It is well known that in the males of all 

 mammals, including man, rudimentary mammae exist. These in several 

 instances have become well developed, and have yielded a copious supply 

 of milk. Their essential identity in the two sexes is likewise shewn by their 

 occasional sympathetic enlargement in both during an attack of the mea- 

 sles. The vesiciila prostatica, which has been observed in many male mam- 

 mals, is now universally acknowledged to be the homologue of the female 

 uterus, together with the connected passage. It is impossible to read Leuck- 

 art's able description of this organ, and his reasoning, without admitting 

 the justness of his conclusion. This is especially clear in the case of those 

 mammals in which the true female uterus bifurcates, for in the males of 

 these the vesicula likewise bifurcates. 



The bearing of the three great classes of facts now given is unmistak- 

 able. But it would be superfluous fully to recapitulate the line of argument 

 given in detail in my Origin of Species. The homological construction of 

 the whole frame in the members of the same class is intelHgible, if we admit 

 their descent from a common progenitor, together with their subsequent 

 adaptation to diversified conditions. On any other view, the similarity of 

 pattern between the hand of a man or monkey, the foot of a horse, the 

 flipper of a seal, the wing of a bat, &c., is utterly inexplicable. It is no scien- 

 tific explanation to assert that they have all been formed on the same ideal 

 plan. With respect to development, we can clearly understand, on the 

 principle of variations supervening at a rather late embryonic period, and 

 being inherited at a corresponding period, how it is that the embryos of 

 wonderfully different forms should still retain, more or less perfectly, the 

 structure of their common progenitor. No other explanation has ever been 

 given of the marvellous fact that the embryos of a man, dog, seal, bat, rep- 

 tile, etc., can at first hardly be distinguished from each other. In order to 

 understand the existence of rudimentary organs, we have only to suppose 

 that a former progenitor possessed the parts in question in a perfect state, 

 and that under changed habits of life they became greatly reduced, either 

 from simple disuse, or through the natural selection of those individuals 

 which were least encumbered with a superfluous part, aided by the other 

 means previously indicated. 



