866 LIFE : OUTLINES OF GENERAL BIOLOGY 



cells, and occurring in asexual, hermaphrodite, and unisexual 

 phases. Klein records no less than twenty-four different forms of 

 V. aureus from the purely vegetative and asexual to the partheno- 

 genetic, for there may be almost entirely male colonies, almost 

 entirely female colonies, and other interesting transitional stages. 

 Klein has also succeeded to some extent in showing that the occur- 

 rence of the various reproductive types depends on outside influences. 



INTERNAL CAVITIES —One of the obvious differences between 

 typical plants and t3rpical animals is the absence of weU-defined 

 internal cavities in the former. No doubt there are air-cavities in 

 leaves and capillary vessels in wood, but apart from a few highly 

 specialised cases like Pitcher-plants, there is nothing that can be 

 compared to stomach, lungs, or heart, or other internal cavities 

 characteristic of animals. Plants are in this respect vaguely compar- 

 able to sponges, where there is no food-canal in the strict sense, its 

 place being taken by the usually intricate system of inhalant and 

 exhalant canals. 



From Hydra and its relatives onwards there is a food-canal with 

 a mouth, a distinct step as compared with a colony of Protozoa, 

 where the food-capture is on the surface. But given a gut or enteron, 

 lined by cells which are the first to be fed, we can readily picture 

 the disproportionate increase of this internal cavity and the origin 

 of folds and pouches, which afford opportunity for all sorts of division 

 of labour. It is interesting to picture the long succession of gut- 

 outgrowths in, say, Vertebrate animals — the gill-pouches, the 

 thyroid diverticulum, the swim-bladder or the lungs, the liver, the 

 pancreas, various caeca, and the allantois. It has been a character- 

 istic line of advance — to increase internal surfaces. The possibilities 

 include more than an increase in the number of different pouchings; 

 there may be great increase in the internal surface of an individual 

 pouch. Thus the lungs of birds, though small and inexpansible, are 

 particularly effective lungs, and partly because of the great increase 

 in their branching internal surface, not to speak of the extensions 

 of air-sacs into the body-cavity, sometimes into the very bones. 



When animals left the water and began to colonise dry land, one 

 of their difficulties must have been that while oxygen was more 

 abundant, it was less available, especially when the surface of the 

 animal became protected by cuticle and shell and the like. One of 

 the ways of meeting this difficulty was the enormous increase of 

 internal air-conducting surfaces, as seen in the tracheae of insects 

 and their relatives. This requires fuller illustration. 



Increase of Surface. — Part of the secret of an average insect's 

 great activity is surely to be foimd in the fact that the blood does n<fl{ 

 become appreciably impure. This is because the blood is alwaj^' 

 near some branch of the system of air-tubes or tracheae, whic 



