GREAT STEPS IN ORGANIC EVOLUTION 867 



ramify into every organ and corner of the insect's body. In most 

 other animals the blood goes to the air, either on the skin or 

 on the gills, or on the walls of the lungs ; but in insects the air goes 

 to the blood. These air-tubes have a very large internal surface, 

 and interchange of gases with the blood is thus facilitated. Simi- 

 larly, as above mentioned, birds' lungs have a very large internal 

 surface on which the blood-vessels are spread out. The feathery gills 

 of the lobster have likewise a very large external surface on which 

 the blood is exposed to aeration, as on a country with a big coast- 

 line. The absorption of digested food from the small intestine is 

 facilitated by the immense surface afforded by the microscopic 

 finger-like processes or villi which line the interior. People who go 

 to live at hill stations or at Johannesburg (about 6,000 feet above the 

 sea) mostly soon show a great increase in the number of their red 

 blood corpuscles. This is a very useful adaptation; for it means 

 that at altitudes where oxygen becomes appreciably scarcer there 

 is compensatory increase in the aggregate surface of the oxygen- 

 capturing red blood corpuscles. 



In breaking up the soil, whether this is effected by ploughing or 

 clod-crushing, or whether the agency be earthworms or frost, is not 

 the result an increase of surface in the soil, or among the soil frag- 

 ments, an increase which promotes, to take the simplest issue, the 

 solution of salts, thus affording more food for the roots of plants? 

 In increase of surface, again, is the significance of the multitudinous 

 leaves of the grasses, which do not get in one another's way, and the 

 value of cut-up leaves. A large tree may be exposing a leaf surface 

 of more than an acre. And even this is understating the fact, since 

 the essential photosynthetic process takes place in the spacious 

 interior of the leaf. For here again the amount of surface counts 

 for much, not only because that of the innumerable cell-walls is very 

 important, but because the starch-grains may be seen forming within 

 the minute chlorophyll bodies, which again offer an exceedingly 

 large surface to light. 



Perhaps a critic may say we are over-emphasising the superficial; 

 yet we cannot but press the simple thesis that one of the great lines 

 of organic evolution — from the colloidal amoeba to the cerebral 

 cortex of man — has been to — and through — more surface. 



BILATERAL SYMMETRY.— It was in some simple worm-types, 

 like Planarians, that bilateral symmetry was first firmly established 

 among multicellular animals; and it was undoubtedly a great step 

 in evolution. Except in a few cases, like adult sea-urchins, it was not 

 afterwards departed from; for even in the large class of snails or 

 Gasteropods, the radical lop-sidedness is a disturbed bilaterahty. 

 A radially symmetrical animal like a jellyfish can move indifferently 

 in any direction in the sea; and though the concave surface of its 



