8 BEPOET — 1881. 



made to assume the winter dress ; it is, on the contrary, far more difficult 

 to change the winter into the summer colouring. 



In some cases — as for instance in the very curious Leptodora crystallina 

 (a fresh-water crustacean, inhabiting deep lakes and reservoirs, and 

 which, as its name denotes, is almost perfectly transparent) — though the 

 two forms are almost exactly similar iu their mature state, the mode of 

 development is very different ; for, while the winter form goes through a 

 well-marked metamorphosis, in the sammer-brood the development is 

 direct. 



It might seem that such enquiries as these could hardly have any- 

 practical bearing. Yet it is not improbable that they may lead to \ery 

 important results. For instance, it Avould appear that the fluke which pi'o- 

 duces 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 cellular tissue in certain 

 parts of animals was known, but Caspar F. Wolff's brilliant memoir had 

 been nearly forgotten ; 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 chieflj^, 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 or- 

 ganic elementary tissues. 



As regards desci'iptive biology, by far the greater number of species 

 now recorded have been named and described within the last half-cen- 

 tury, and it is not too much to say that not a day passes without add- 

 ing new species to our lists. A comparison, for instance, of the edition 

 of Cuvier's ' Regno Animal,' published in 1828, as compared with the 

 present state of our knowledge, is most striking. 



Dr. Giinther has been good enough to make a calculation for me. 

 The numbers, of course, are only approximate, but it appears that while 

 the total number of animals described up to 1831 was not more than 

 70,000, the number now is at least 320,000. 



Lastly, to show how large a field still remains for exploration, I 

 may add that Mr. Waterhouse assumes that our museums contain not 

 fewer than 12,000 species of insects which have not yet been described, 

 while our collections do not probably contain anything like one-half 

 of those actually in existence. Further than this, the anatomy and habits 

 even of those which have been described offer an inexhaustible field for 



