12 LECTURES ON BIOLOGY 



a stronger development of these parts. Similarly one might 

 explain with more or less probability of truth the origin of most 

 organs. 



On the other hand, as we have already seen, disuse leads to 

 reversion and degeneration of the affected parts. If an organ 

 was deprived of work owing to altered conditions of life, gradual 

 degeneration followed of necessity. Thanks to the conservatism 

 of heredity, the organs may for some time be retained in the 

 successors, but finally the last .vestige is lost. The numerous 

 rudimentary organs form ample illustrations of this phenomenon. 

 We need only mention, among many other similar instances, 

 the loss of eyes in many cave-animals, the degenerate eyes of 

 the mole, the breast glands of male animals, and the muscle of 

 the human ear, whereby it can be pulled upwards or twitched 

 forward or backward. 



In all nature there is hardly another case in which one may 

 so clearly see the adaptation of an organ to its functions and 

 the improving or degenerating influence of use and disuse as 

 in the development of the arms and legs of the vertebrates. 

 However different the extremities of vertebrates may externally 

 appear, it is possible to demonstrate, if not in the adult, at 

 any rate in the embryonic stage, that on the whole they possess 

 similar bones and are thus proved to be of common origin. 



Fishes represent the lowest stage of the vertebrates. In 

 these denizens of the water the extremities have developed into 

 instruments of rowing, or fins. These are either paired or 

 unpaired. We know that the unpaired extremities are derived 

 from a continuous dorsal fringe which commences immediately 

 behind the head and continues to the anus. In earliest youth 

 this uniform fringe divides itself in most fishes into dorsal and 

 anal fins, and the great locomotor fin of the tail. In a similar 

 manner the paired fins originate from two at first continuous 

 lateral folds which divide later into the two anterior pectoral and 

 ventral fins. Soon afterwards solid skeleton parts appear in the 

 skin folds, giving to the whole the necessary stability. Of these 

 two kinds of extremities, the unpaired are phylogeneticaily the 

 earlier, for we find them already in the lower types of fishes, the 

 primitive fish Branchiostoma (Amphioxus), and in the Lampreys 

 (Cyclostoma), which are as yet without paired fins. Being 



