14 ADDRESS TO THE BRITISH ASSOCIATION, 1881. 



metamorphosis, in the summer brood the development 

 is direct. 



It might seem that such enquiries as these could 

 hardly have any practical bearing. Yet it is not im- 

 probable that they may lead to very important results. 

 For instance, it would appear that the fluke which 

 produces the rot in sheep, passes one phase of its 

 existence in snails or slugs, and we are not without 

 hopes that the researches, in which our lamented friend 

 Prof. Rolleston was engaged at the time of his death, 

 and which Mr. Thomas is continuing, will lead, if not to 

 the extirpation, at any rate to the diminution, of a pest 

 from which our farmers have so grievously suffered. 



It was in the year 1839 that Schwann and Schleiden 

 demonstrated the intimate relation in which animals 

 and plants stand to each other, by showing the identity 

 of the laws of development of the elementary parts in 

 the two kingdoms of organic nature. Analogies indeed 

 had been previously pointed out ; the presence of cel- 

 lular tissue in certain parts of animals was known, bat 

 Caspar F. Wolff's brilliant memoir had been nearly for- 

 gotten ; and the tendency of microscopical investigation 

 had rather been to encourage the belief that no real 

 similarity existed; that the cellular tissue of animals 

 was essentially different from that of plants. This had 

 arisen chiefly, perhaps, because fully formed tissues were 

 compared, and it was mainly the study of the growth of 

 cells which led to the demonstration of the general law 

 of development for all organic elementary tissues. 



As regards descriptive biology, by far the greater 

 number of species now recorded have been named and 

 described within the last half-century, and it is not too 



