xviii THE ANIMAL KINGDOM. 



archaic types, the survivors of a large group of Devonian forms. They were called 

 by Agassiz prophetic types, as they pointed to the coming of more highly wrought, 

 specialized forms, the amphibians. They are now regarded as ancestral forms from 

 which have originated two lines of organisms, one culminating in the bony fishes, and 

 the other in the labyrinthodouts and salamanders and other batrachians. 



The king or horse-shoe crab {Lirmdus) is likewise a composite, synthetic, compre- 

 hensive or generalized type, as it is variously called. Its development and structure 

 shows certain features closely resembling the Arachnida, while it is also closely allied 

 by other points to the Crustacea, with other features peculiar to itself. Its position in 

 the scheme of nature is now in dispute, owing to the aduiixture of characters in 

 which it resembles both the Crustacea and Arachnida. Now Limulus is a survivor or 

 remnant of a long line of forms which flourished in the palaeozoic seas, at a time when 

 there were no genuine Crustacea nor Arachnida, which did not arise until long after 

 the Merostomata {Eurypterus, etc.) and their allies, the trilobites, began to disappear. 

 These generalized, composite arthropods lived, had their day, and finally gave way to 

 the hosts of highly specialized modern Crustacea, such as the shrimps and crabs of our 

 seas, and to the scorpions and spiders inhabiting the land, and which owe their diver- 

 sity of form to the highly wrought structure of a few special parts. 



So, among insects, the earlier were the more generalized, old-fashioned forms. Such 

 were the white ants, cockroaches, grasshoppers, may-flies and dragon-flies, the earliest 

 insects known. Their numbers are scanty at the present day. They have been ui 

 part supplanted by the thousands of species of beetles, moths, butterflies, ants, wasps, 

 and bees, so characteristic of the present age of the world as compared with the insect 

 life of the carboniferous period. 



Among mammals the horse is the most specialized ; its generalized ancestors were 

 the Coryphodon and Eohijyjnts. The latter had four usable toes, and the rudiments 

 of a fifth on each forefoot, and three toes behind. The history of the horse family is 

 a record of successive steps by which a highly specialized type was produced, culmina- 

 ting in that extreme form, the American trotting horse, which can only do one thing 

 well, i. e. excel in trotting over a racecourse. 



PHYSIOLOGY. 



The most diflicult line of study in biology is to determine how the organs do their 

 work. This is the ofiice of the physiologist. It is comparatively easy to discover how 

 a fish uses its fins in swimming, how a mammal walks, how a bird flies ; but it is difli- 

 cult to ascertain how the internal organs perform their functions ; how the stomach 

 digests food, exactly what oflice the biliary and pancreatic fluids fill in the compli- 

 cated process of digestion ; the function of the spleen, and so on with the other viscera. 

 Here, besides observation and comparison, the physiologist has to rely on experiments 

 to test the results of his observations. The pathway to a complete understanding of 

 human physiology lies through the broad and as yet only partially surveyed field 

 of animal physiology. We can better understand the physiology of digestion in man 

 by studying the process of intracellular digestion in the Infusoria, the sponges, and 

 jelly-fish, where, owing to the transparency of the tissues, the process can be actually 

 observed ; so likewise, the nature of muscular movements can best be understood by 

 observing under the microscope the contractility of the protoplasm of individual 

 cells or one-celled organisms. The physiology of reproduction could never have been 



