Differentiation. 291 



times it exists only above and below, and allows the two vaginae to communicate with 

 each other about the middle or near the neck ; most frequently, as remarked by Majoc- 

 chi, Boehmer, Cassan, and others, it does not reach to the vulva, and, further, itis, in 

 general, only the continuation of a similar disposition of the womb." ( Velpeau's Mid- 

 wifery, 115.) 



The normal and usual change which takes place in the lungs at birth 



afford a first-class example of the modifying effect of an environing force 

 upon an organic machine fitted to be moved by it. Before birth the 

 lungs, which have been built up by heredity, according to a certain pat- 

 tern, weigh one seventieth ( ^ ) of the whole body. They are very small, 

 of a "compact, heavy, granular, yellowish-pink, gland-like substance," 

 with specific gravit}^ heavier than water, and absolute weight of about 

 1^- ounces. The admission of air to the lungs immediately changes their 

 substance into a loose, light, rose-pink, spongy structure, their absolute 

 weight rises to as much as 2^ ounces, and relative to the body ^ or ^U., 

 and the specific gravity sinks to one-third that of water, their size cor- 

 respondingly expanding. ( Quain. ) The changes wrought by the air 

 in this organ in a few hours almost equal those of all the previous 

 months. There appears to be a sort of preparatory adaptation of the 

 young animal to this sudden exposure to the direct impact of a force 

 which had before affected it indirectly. 



A good many examples of retrogressive adaptation have been given, 

 enough to prove the influence of the environment in effecting adaptations, 

 even if it has to cause an " advance backward " in or- 

 der to accomplish it. The limb of the Dolphin shown 

 in fig. 120, is a case in point. The thumb and the 

 fourth and fifth fingers are relatively greatly reduced, 

 and so are the arm bones in respect to their length. 

 This animal is descended from a land mammal and has 

 become adapted to a sea life. But though reduced as 

 to legs, the dolphin has a highly developed brain. 



FIG. 120. Right fore limb of 

 Dolphin. 



h. Hurnerus. 



r. Radius. 



u. Ulna. 



1. Thumb. 



2, 3, 4, 5. Fingers. 



p. Pisiform bone. Compare 

 with fig. 69. 

 ' ( VanBeneden & Gervais. ) 



FIG. 121. Foot of Night-jar 

 or Goat-sucker, showing its tine 

 comb. The same sort of an ap- 

 pendage is developed on the foot 

 of the common Barn Owl and 

 the Heron and Bittern species. 

 These are all lousy birds, a fact 

 which suggests the use and 

 causes of the development of the 

 instrument. ( After Owen. ) 



FIG. 120. FIG. 121. 



The pisiform bone shown in this figure, and several others, is a carpal 



bone said to have originally belonged to a sixth finger. We have 



