96 SCIENCE IN THE LAST HALF CENTURY. 



same way with diphtheria, typhoid and scarlet fever. To one who has 

 seen half a street swept clear of its children, or lias lost his own by 

 these horrible pestilences, passing one's offspring through the tire to 

 Moloch seems humanity comi)ared with the proposal to deprive them of 

 half their chances of health and life because of the discomfort to dogs 

 and cats, rabbits and frogs, which may be involved in the search for 

 means of guarding them. 



EXPLORATION. 



An immense extension has been effected in our knowledge of the dis- 

 tribution of plants and animals; and the elucidation of the causes which 

 liave brought about that distribution has been greatly advanced. The 

 establishment of meteorological observ^ations by all civilized nations, 

 lias furnished a solid foundation to climatology ; while a growing sense 

 of the importance of the influence of the " struggle for existence " aflbrds 

 a wholesome check to the tendency to overrate the influence of climate 

 on distribution. Expeditions, such as thatof the Challenger, equii)ped, 

 not for geographical exploration and discovery, but for the purpose of 

 throwing light on problems of physical and biological science, have 

 been sent out by our own and other Governments, and have obtained 

 Stores of information of the greatest value. For the first time, we are 

 in possession of something like precise knowledge of the physical feat- 

 ures of the deep seas, and of the living population of the floor of the 

 ocean. The careful and exhaustive study of the phenomena i)resented 

 by the accumulations of snow and ice, in polar and mountainous regions, 

 which has taken place in our time, has not only revealed to the geologist 

 an agent of denudation and transport, which has slowly and quietly 

 produced effects, formerly confidently referred to diluvial catastrophes, 

 but it has suggested new methods of accounting for various puzzling- 

 facts of distribution. 



PALEONTOLOGY. 



Palaeontology, which treats of the extinct forms of life and their suc- 

 cession and distribution upon our globe, a branch of science which 

 could hardly be said to exist a century ago, has undergone a wonderful 

 development in our epoch. In some groups of animals and plants the 

 extinct representatives, already known, are more numerous and impor- 

 tant than the living. There can be no doubt that the existing Fauna 

 and Flora is but the last term of a long series of equally numerous con- 

 temporary species, which have succeeded one another, by the slow and 

 gradual substitution of species for species, in the vast interval of time 

 >yhich has elapsed between the deposition of the earliest fossiliferous 

 ^trata and the present day. There is no reasonable ground for believ- 

 ing that the oldest remains yet obtained carry us even near the begin- 

 nings of life. The impressive warnings of Lyell against hasty specula- 

 iious, based iaj)ou negative evidence, have l)eeu fully justified; time 



