282 Proceedings of the Boyal Physical Society, 



the second quarter of this century, which represented the 

 simple cell as the lowest form of life, was justly regarded as 

 one of the greatest physiological discoveries of the age ; and, 

 although germs and protoplasm have since, to a certain 

 extent, shared in this distinction, its function in the con- 

 struction of the tissues still holds good. The role of cells in 

 the elaboration of the bodies of animals and plants may be 

 compared to that of bricks in a building, with this difference, 

 however, that the former assume a great variety of forms — 

 fusiform, caudate, stellate, squamous, ciliated, epithelial, 

 etc., according to their position and function in the animal 

 economy. 



Continuing our general survey of the organic world, 

 it may be furtlier observed that between the unicellular 

 organisms and the higher animals, there are numbers of 

 intermediate forms more or less differentiated ; and so 

 gradual is their differentiation manifested, that the whole 

 series may be compared to a chain having for one of its 

 terminal links a unicellular organism and for the other man 

 himself. But yet all these links are absolutely distinct 

 and independent of each other, except on the lines of 

 their genetic connection. 



Another remarkable fact is, that the present-day species 

 are never precisely similar to their fossil ancestors, from 

 which it follows that some modification in the somatic 

 structure of the former has taken place in the course of 

 time ; and it is significant that the extent of this modifica- 

 tion corresponds, generally, with the time that has elapsed 

 since the latter were in life. But this change has not been 

 at a uniform rate in the different species, some having 

 adhered so persistently to their peculiar structure and habits 

 that they continued to flourish through several geological 

 periods without having undergone any material modifica- 

 tion. For example, the Lampshells, King- Crabs, Pearly 

 Nautilus, and many others, have come down from Pala30zoic 

 times to the present day with comparatively little change 

 during these millions of years. Indeed, so conservative 

 were some of them that they preferred to die out rather 

 than change their antiquated ways. 



