EVOLUTION 



3032 



EVOLUTION 



Nor was the step primarily one 

 of increasing complexity either 

 of organization or activity, for 

 many ciliated infusorians, though 

 unicellular, are far more complex 

 in plasmic architecture and in 

 ways of life than the fresh-water 

 polyps, built up of thousands of 

 cells. The step was on to a new 

 line of organization, the formation 

 of a many-celled body in which 

 scope was given to division of 

 labour among the component 

 units. The structural side of this 

 is called differentiation. The at- 

 tainment of a multicellular body 

 opened the way to unlimited 

 specialisation of function, and 

 also to an increase of size, which, 

 other things being equal, counts 

 for something in a rough and cal- 

 lous physical environment. The 

 nemesis of this great step of gain- 

 ing a body was apparently that 

 organisms became liable to na- 

 tural death in proportion to the 

 complexity of the bodily frame- 

 work. For natural death appears 

 to result from the accumulation 

 of wear and tear effects, and the 

 failure of the ceaseless attempts to 

 cope with these. 



Evolution of Sex 



Another big step was the evolu- 

 tion of male and female multi- 

 cellular individuals within the same 

 species, the two sexes being comple- 

 mentary in the process of reproduc- 

 tion which secures the continuance 

 of the race. The biological signifi- 

 cance of the evolution of sex among 

 multicellular animals was threefold. 

 First, sexual reproduction implies 

 that multiplication is effected by 

 the liberation of germ-cells, which 

 is more economical than separat- 

 ing off fragments or buds. There 

 is also an increasing possibility 

 of a large number of offspring. 

 Secondly, to have special germ- 

 cells in some measure apart from 

 the body-cells tends to secure the 

 hereditary persistence of a success- 

 ful constitution, and lessens the 

 risk of the offspring being pre- 

 judiced by disadvantageous dints 

 made on the parent's body. 



Thirdly, to have two different 

 kinds of sex-cells, which have to 

 unite at the beginning of each in- 

 dividual life, offers opportunities 

 for new permutations and combi- 

 nations of qualities, for those new 

 departures technically called varia- 

 tions and mutations. The separa- 

 tion of sperm -producers or males 

 and egg-producers or females, 

 which differ deeply in constitu- 

 tion, would also tend to increase 

 the range of cross -fertilisation 

 which is often advantageous, and 

 would permit of a profitable divi- 

 sion of labour between the parents 

 in their relations to the offspring. 



Differentiation includes a multi- 

 tude of evolutionary steps. In the 

 creature called Volvox, which con- 

 sists of a thousand or ten thousand 

 flagellate green cells united in a 

 ball, all the component units,except 

 those concerned with reproduction, 

 are alike. There is no division of 

 labour in the colony. In sponges, 

 however, we see the beginnings of 

 tissues, i.e. groups of similar cells 

 performing the same functions. 

 Thus contractile tissue, connective 

 tissue, and flagellate lining tissue 

 (or epithelium) appear among 

 sponges. In the next great series of 

 animals, the Coelentera or Stinging 

 animals, other kinds of tissue, such 

 as nervous and glandular, are 

 differentiated, and we find the first 

 occurrence of organs, such as sen- 

 sory, digestive, and reproductive 

 organs. 



In most sponges and stinging ani- 

 mals the symmetry of the body is 

 radial, i.e. there is no right or left 

 side ; the animal is the same all 

 round. This is well suited to a se- 

 dentary or drifting existence, but 

 for more strenuous life involving 

 the pursuit of prey and mates, and 

 the avoidance of enemies, bilateral 

 symmetry, which virtually began 

 among " worms," is incomparably 

 more effective. It implies a right 

 and a left side, a head end which 

 leads the way, and a tail end. 



With the acquisition of bilateral 

 symmetry was associated the estab- 

 lishment of an anterior brain and 

 the development of a head worthy 

 of the name. This opened up an- 

 other line of advance, technic- 

 ally called integration, in contrast 

 to differentiation. Differentiation 

 means increasing complexity of 

 parts, integration means their more 

 perfect unification and control, 

 and one of the main functions of 

 the nervous system is integrative. 

 Differentiation and Integration 



The story of evolution, apart 

 from retrogressive parasites and 

 other degenerates, is one of pro- 

 gressive differentiation and inte- 

 gration, and the evolutionist has to 

 record a long series of achieve- 

 ments. Among these are : an open 

 food canal; a body cavity or coelom 

 between the food canal and the 

 body wall ; striped or swiftly con- 

 tracting muscle ; a circulatory sys- 

 tem for distributing digested food 

 and oxygen throughout the body 

 and for collecting waste; oxygen- 

 capturing pigments such as haemo- 

 globin ; a segmented body as in 

 earthworms ; a renewable external 

 armour as in crustaceans; muscular 

 appendages first unjoin ted and 

 then jointed; specialised sense 

 organs such as eyes and balancers , 

 improved respiratory arrangements 

 reaching extraordinary perfec- 



tion among insects ; delicate adjust- 

 ments for filtering out the poison- 

 ous nitrogenous waste of the body. 



We can only allude to the estab- 

 lishment of the leading types of 

 architecture represented by the 

 various series of invertebrates or 

 backboneless animals. Besides the 

 sponges and coelenterates, we have 

 to deal with the great variety of 

 worm-types ; with the higher seg- 

 mented worms or Annelids ; with 

 the starfishes, sea-urchins, and the 

 like forming the Echinoderms ; 

 with the jointed-footed Arthro- 

 pods, such as crustaceans, insects, 

 and spiders ; with the unsegmented 

 molluscs without appendages, such 

 as bivalves, snails, and cuttles; 

 and with many smaller groups. 

 Origin of Vertebrates 



A step of great magnitude was 

 the origin of the backboned ani- 

 mals or Vertebrates. It is not un- 

 likely that these emerged from the 

 stock of segmented worms. Their 

 origin meant a fresh start on a new 

 line of more masterful life. A dom- 

 inant feature was the establish- 

 ment of a relatively large brain 

 protected by a skull, and of a long, 

 spinal cord protected by the back- 

 bone. Of great importance also 

 was the first appearance of bone 

 and of an internal living skeleton 

 (usually of bone) pervading the 

 whole body, and contributing to 

 integration. In the establishment 

 of numerous glands of internal se- 

 cretion, whose hormones or regula- 

 tive substances are distributed by 

 the blood throughout the body, 

 a chemical integration began to 

 operate, or to do so on a larger scale. 



Skulls began with the hags and 

 lampreys ; jaws and paired fins, 

 scales and typical gills with the 

 true fishes ; digits, true lungs, vocal 

 chords, and a mobile tongue with 

 Amphibians ; the antenatal robes 

 (or foetal membranes) known as 

 amnion and allantois with the rep- 

 tiles ; a four-chambered heart with 

 the crocodilians ; warm-blooded- 

 ness, or keeping the temperature of 

 the body approximately constant, 

 with birds and mammals, which 

 also show an enormous advance in 

 brain development ; the usually 

 prolonged antenatal connexion be- 

 tween mother and offspring with 

 the placental mammals. And just 

 as amphibians mark the transition 

 from water to dry land, so the ex- 

 tinct flying dragons (Pterodactyls) 

 pointed towards that mastery of 

 the air which birds and bats have 

 attained. 



Along with the great structural 

 advances, there went a functional 

 progressiveness. The smooth work- 

 ing that marks even the simplest 

 creatures is not lost with intricate 

 organization. But the scope of the 



