A HALF-CENTURY OF SCIENCE. 71 



which descends to the ground, and makes small galls on the roots of 

 the oak. From these emerge an insect known as Biorhiza aptera, 

 which again gives rise to the common oak-apple. 



It might seem that such inquiries as these could hardly have any 

 practical bearing. Yet it is not improbable 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 

 the black slug ; and we are not without hopes that the researches, in 

 which our lamented friend Professor Rolleston was engaged at the 

 time of his death, which we all so much deplore, 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. 



As regards descriptive biology, by far the greater number of spe- 

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

 century. Dr. Gtinther 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 estimates that the British Museum 

 alone contains not fewer than 12,000 species of insects which have not 

 yet been described, while our collections do not probably contain any- 

 thing 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 research, and it is not going too far to say 

 that there is not a single species which would not amply repay the 

 devotion of a lifetime. 



One remarkable feature in the modern progress of biological sci- 

 ence has been the application of improved methods of observation and 

 experiment, and the employment in physiological research of the exact 

 measurements employed by the experimental physicist. Our micro- 

 scopes have been greatly improved. The use of chemical reagents in 

 microscopical investigations has proved most instructive, and another 

 very important method of investigation has been the power of obtain- 

 ing very thin slices by imbedding the object to be examined in paraffine 

 or some other soft substance. In this manner we can now obtain, say, 

 fifty separate sections of the egg of a beetle or the brain of a bee. 



At the close of the last century Sprengel published a most sugges- 

 tive work on flowers, in which he pointed out the curious relation exist- 

 ing between these and insects, and showed that the latter carry the 

 pollen from flower to flower. His observations, however, attracted 

 little notice until Darwin called attention to the subject in 1862. It 



