PRESIDENTIAL ADDRESS. O 



transition is actually presented, during the successive stages of its 

 growth. For it begins life as a Cornuspira, .... its shell forming a 

 continuous spiral tube, with slight interruptions at the points at which 

 its successive extensions commence ; while its sarcodic body consists of 

 a continuous coil with slight constrictions at intervals. The second 

 stage consists in the opening out of its spire, and the division of its 

 cavity at regular intervals by transverse septa, traversed by separate 

 pores, exactly as in Peneroplis. The third stage is marked by the 

 subdivision of the " peneropline " chambers into chamberlets, as in the 

 early forms of Orbiculina. And the fourth consists in the exchange 

 of the spiral for the cyclical plan of growth, which is characteristic of 

 Orbitolites ; a chculat* disc of progressively increasing diameter being 

 formed by the addition of successive annular zones around the entire 

 periphery.' 



The shells both of Foraminifera and of Mollusca afford peculiarly 

 Instructive examples for the study of recapitulation. As growth of 

 the shell is effected by the addition of new shelly matter to the part 

 already existing, the older parts of the shell are retained, often 

 unaltered, in the adult ; and in favourable cases, as in Orbitolites 

 tenuissima, all the stages of development can be determined by simple 

 inspection of the adult shell. 



It is important to remember that the Eecapitulation Theory, if 

 valid, must apply not merely in a general way to the development of the 

 animal body, but must hold good with regard to the formation of each 

 organ or system, and with regard to the later equally with the earlier 

 phases of development. 



Of individual organs the brain of birds has been already cited. The 

 formation of the vertebrate liver as a diverticulum from the alimentary 

 canal, which is at first simple, but by the folding of its walls becomes 

 greatly complicated, is another good example ; as is also the develop- 

 ment of the vomer in Amphibians as a series of toothed plates, equiva- 

 lent morphologically to the placoid scales of fishes, which are at first 

 separate, but later on fuse together and lose the greater number of 

 their teeth. 



Concerning recapitulation in the latter phases of development and 

 in the adult animal, the mode of renewal of the nails or of the 

 epidermis generally is a good example, each cell commencing its 

 existence in an indifferent form in the deeper layers of the epidermis, 



