164 THE POPULAR SCIENCE MONTHLY. 



8. Development within the Chrysalis Shell. — It is well 

 known that many insects emerge from the egg not in their final form, 

 but in a wormlike form, called a larva. After this they pass into a 

 second passive state, in which they are again covered with a kind of 

 shell — a sort of second egg-state, called the chrysalis. From this 

 they again emerge as the perfect insect. The butterfly is the most 

 familiar, as well as the best illustrated, of these changes. The larva 

 or caterpillar eats with enormous voracity, and grows very rapidly. 

 When its growth is complete, it covers itself with a shell, and remains 

 perfectly passive and almost immovable for many days or weeks. 

 During this period of quiescence of animal functions there are, how- 

 ever, the most important changes going on within. The wings and 

 legs are formed, the muscles are aggregated in bundles for moving 

 these appendages, the nervous system is more highly developed, the 

 mouth, organs, and alimentary canal, are greatly changed and more 

 highly organized, the simple eyes are changed into compound eyes. 

 Now, all this requires expenditure of force, and therefore decomposition 

 of matter ; but no food is taken, therefore the chrysalis must consume 

 its own substance, and therefore lose weight. It does so ; the weight 

 of the emerging butterfly is in many cases not one-tenth that of the 

 caterpillar. Force is stored up in the form of organic matter only to 

 be consumed in doing plastic work. 



9. Mature Animals. — Whence do animals derive their vital 

 force ? I answer, from the decomposition of their food and the de- 

 composition of their tissues. 



Plants, as we have seen, derive their vital force from the decom- 

 position of their mineral food. But the chemical compounds on which 

 plants feed are very stable. Their decomposition requires a peculiar 

 and complex contrivance for the reception and utilization of sunlight. 

 These conditions are wanting in animals. Animals, therefore, cannot 

 feed on chemical compounds of the mineral kingdom ; they must have 

 organic food, which easily runs into decomposition ; they must feed on 

 the vegetable kingdom. 



Animals are distinguished from vegetables by incessant decay in 

 every tissue — a decay which is proportional to animal activity. This 

 incessant decay necessitates incessant repair, so that the animal body 

 has been likened to a temple on which two opposite forces are at work 

 in every part, the one tearing down, the other repairing the breach as 

 fast as made. In vegetables no such incessant decay has ever been 

 made out. If it exists, it must be very trifling in comparison. Proto- 

 plasm, it is true, is taken up from the older parts of vegetables, and 

 these parts die ; but the protoplasm does not seem to decompose, but 

 is used again for tissue-building. Thus the internal activity of ani- 

 mals is of two kinds, tissue-destroying and tissue-building ; while that 

 of plants seems to be, principally, at least, of one kind, tissue-building. 

 Animals use food for force and repair and growth, and in the mature 



