492 SCIENCE PROGRESS. 



very largely on the stimulation applied to them in the 

 struggle for existence in successive generations of indi- 

 viduals. In the larva of the frog the limbs and fin-rays 

 ceased to be stimulated by exercise, and consequently 

 developed less and less in this period of life. In adult 

 terrestrial life, on the other hand, the paired limbs were 

 constantly used, and the stimulation thus caused would 

 produce its effect at the corresponding period of life in the 

 development of the limbs in the adult form at the end of 

 the larval period. 



We may next consider recapitulation in the develop- 

 ment of fishes. A classical instance of this is the 

 development of the tail in certain Teleosteans. The 

 primitive condition of the tail is the symmetrical condition, 

 in which the fin membrane is continued round the end of 

 the notochord, and supported internally by cartilaginous 

 rays, outside which, beneath the epidermis, are very 

 numerous thin horny fibrous rays uniformly close together. 

 This condition existed in the earliest Elasmobranchs, for 

 instance in Pleui^acanthus. In the majority of Selachians 

 from the Carboniferous to the present day, without any 

 fundamental modification of the character of the skeletal 

 ray-supports, there is an alteration of the shape of the fin. 

 The ventral portion is enlarged to a greater or less degree 

 in comparison with the upper, and so forms a ventral lobe 

 of the fin, the original terminal portion forming the dorsal 

 lobe. 



In the osseous fishes, which general term includes 

 Ganoids, Teleosteans and Dipnoi, there is an important 

 modification of the skeleton which extends to the skeleton 

 of the tail. Dermal calcifications derived from the scattered 

 tubercles of the Elasmobranchs unite to form bony fin-rays 

 outside the horny fibres. These at the bases of the rays 

 correspond in position to the cartilaginous median rays, 

 while distally they branch and correspond more with the 

 original horny fibres. With the appearance of this modi- 

 fication the tail may retain its original symmetrical or 

 diphycercal condition as in several Devonian Ganoids, in 

 the modern Polypterus, and in both ancient and modern 



