EVOLUTION 



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EVOLUTION 



Evolution. Early and late stages in evolution. A. Early 

 four-toed ancestor of the horse. Its proportionate size is 

 shown by comparison with a. B. Head and skull of an 

 early ancestor (Meritherium) of the elephant, whose head 

 and skull are shown in b. C. The oldest known bird, 

 Archaeopteryx, two specimens of which have been obtained 

 Irom Jurassic strata, compared with, c, golden eagle 



By courtesy of Andrew Melrose 



The embryological evidence is 

 very striking. The embryos of the 

 higher vertebrates, viz. reptiles, 

 birds, and mammals, develop for a 

 considerable distance along the 

 same road, or along closely parallel 

 roads, before they diverge. For 

 instance, in the neck region of 

 the embryo higher vertebrate, 

 there are gill-clefts or visceral 

 clefts which have no respiratory 

 significance, and can hardly be 

 said to be useful, except that the 

 first becomes the Eustachian tube 

 connecting the ear passage with 

 the back of the mouth. These can 

 only be recapitulations of the re- 

 spiratory apparatus of remote 

 aquatic, lower vertebrate,ancestors. 



Great Steps in Organic Evolution 

 There was a time when the tem- 

 perature of the earth was far too 

 high to admit of the existence of 

 any forms of life like those we 

 know. Whether germs of living 

 organisms reached the earth from 

 elsewhere, or whether very simple 

 living organisms evolved upon the 

 earth out of non-living matter, is 

 unknown, but it is certain that liv- 

 ing organisms did have a beginning 



the first organ- 

 isms were much 

 simpler in organi- 

 zation than any 

 clearly visible 

 living creatures 

 of to-day. It has 

 been suggested 

 that the earliest 

 living beings 

 were minute, pos- 

 sibly ultra-micro- 

 scopic particles 

 of the nature of 

 chromatin, a pro- 

 tein material 

 characteristic of 

 all cell-nuclei. 

 These hypothe- 

 tical primitive 

 organisms have 

 been called bio- 

 cocci. Some of 

 these may have 

 given rise to the 

 bacterial type of 

 organism, con- 

 sisting of a 

 minute globule 

 of chromatin sur- 

 rounded bya firm 

 envelope. As time 

 went on and size 

 increased the 

 chromatin-glo- 

 bules might in- 

 crease in number 

 and acquire some 

 complexity of 

 arrangement,and 

 a non-chromati- 

 nic ground sub- 

 stance (cytoplasm) might accu- 

 mulate around them and within 

 the envelope. 



On another line of evolution a 

 less vegetative and more predatory 

 organism may have arisen by the 

 formation, around a number of 

 biococci or chromatin-grains, of 

 an enveloping matrix of active 

 semi-fluid substance exhibiting 

 streaming or amoeboid movements. 

 This was a prototype of the animal, 

 and it preyed upon other minute 

 creatures. Later on, the chro- 

 matin-grains probably concen- 

 trated to form a definite cell- 

 nucleus in the midst of the active 

 matrix, and a true cell was formed. 

 These suggestions serve to indi- 

 cate that probably a long journey 

 had to be travelled before even the 

 first true cells appeared. 



The next great steps probably 

 consisted in the establishment of 

 numerous distinct types of cellular 

 organization, besides the bacterial 

 and the amoeboid. On the animal 

 line of evolution, towards which 

 the primeval amoeboid organisms 

 pointed, there doubtless arose all 

 sorts of specialisations of the 



upon the earth, and probable that creeping cell, many with support- 



ing skeletal framework. More ac- 

 tive forms had lashes of protoplasm 

 instead of outflowing threads and 

 lobes. Others,with an enclosing cyst, 

 were adapted for spending much 

 of their life as passive parasites. 



One of the primeval great events 

 must have been the emergence of 

 green plants. These perhaps origin- 

 ated among flagellate infusorians 

 on the animal line which had been 

 able to build up the green pigment 

 chlorophyll, the most important 

 substance in the world, next to liv- 

 ing matter itself. The divergence 

 between plants and animals was 

 one of the greatest cleavages in evo- 

 lution. While all typical animals 

 require organic food which has 

 been worked up for them by other 

 living creatures, green plants are 

 able to utilise the energy of the 

 sunlight, shining through a screen 

 of chorophyll, to break down the 

 carbon dioxide of the air, the car- 

 bon being used in the rynthesis 

 of complex organic compounds. 

 Thus green plants feed at a low 

 chemical level, on air, water, and 

 salts, and build up nutritive ma- 

 terials which animals utilise. 



Moreover, the plant ceU is 

 almost always surrounded by an 

 envelope or cell-wall of cellulose, 

 and this restriction, taken along 

 with the poorly developed means of 

 getting rid of nitrogenous waste - 

 products, may explain the fixity 

 and sluggishness of plant-life. We 

 are unable here to follow the evo- 

 lution of the plant world which 

 went on simultaneously with that 

 of the animal world. One of the 

 striking general impressions is 

 that of a succession of dominant 

 groups, each reaching supremacy, 

 and then yielding to another. 

 Thus the gigantic club-mosses and 

 horse-tails which made great 

 forests yielded to Cycad-like forms 

 and passed into relative insigni- 

 ficance ; the Cycadophytes in turn 

 yielded to the flowering plants. 



Multicellular Organisms 

 It was a red-letter day in organic 

 evolution when " bodies " began 

 to be, i.e. when some living crea- 

 tures passed from the unicellular 

 to the multicellular grade of organi- 

 zation. Many flagellate infusorians 

 form colonies or families of con- 

 nected cells, the daughter-units, 

 formed by division of the mother- 

 unit, remaining associated, instead 

 of drifting apart to live isolated 

 lives, and it was probably in some 

 such way that multicellular organ- 

 isms began. It must be clearly 

 understood that the step was not 

 primarily one of increase in size, 

 for a rotifer or wheel-animalcule 

 built up of a thousand cells is much 

 smaller than a unicellular infusor- 

 ian such as the Noctiluca (q.v.). 



