11 



Amongst the MoUusca and Coelcnterata there are scarcely any 

 species invisible to the naked eye, and some arc large, particularly 

 the highly organized Cuttlefishes and other Cephalopoda ; a few 

 of the Bivalves attain to large dimensions. 



But turning from all the groups treated of to the Protozoa W8 

 find nearly all the species, except the Sponges Microscopic. 



The Gregai'inida, Rhizopoda and Infusoria are nearly all quite 

 invisible to the naked eye, and many of the lovrer forms require 

 to be magnified from a thousand to fifteen hundred times to 

 enable an artist to figure them. 



Upon the whole, I think, it has been fairly shown that increase 

 in size is correlated with higher developement, although the fact 

 is much obscured in the higher animals, because natural selection 

 acts upon them Math great force, and it may be very disadvan- 

 tageous to some of the species to be of large size and often even 

 fatal to their existence. Mr. Darwin has shown that during a 

 severe drought, which occurred in South America, the larger 

 Mammalia perished, but the mice survived because the dew fur- 

 nished them with sufficient moisture. 



I think there can be little doubt that large animals, with a 

 complex structnre, are more sensitive to external relations than 

 smaller and more lowly organized creatures ; and, therefore, I am 

 not surprised to find that so many of the largest Mammalia that 

 have once existed on the earth have failed in the struggle for 

 existence and have perished. 



Amongst the highest MoUusca nearly all the genera have 

 become extinct and but very few of the thousands of species 

 remain, by which we are enabled to form some little idea what 

 the extinct species were like. 



One fact, I think, is clearly established, that Microscopic 

 animals are always very lowly organized, and that the lowest forms 

 of animalculae require the highest power of the microscope even 

 to distinguish them at all. 



I have, thus far, endeavoured to show that increase in size is, in 

 animals, correlated with a greater complexity of organization, and 

 I now wish to draw your attention to another rule, which obtains 

 that the most highly organized animals require comparatively the 

 longest periods of time for their development. 



To put the point concisely, I wish to prove that what is true 

 of the development of life in space is true in time. 



Commencing with the lowest Protozoa, the Amoeba;, so short a 



