ANIMAL EVOLUTION 



52-5 



further. From this brief statement it is 

 evident that a butterfly depends for its 

 existence on a very definite correlation and 

 coordination of, and a most delicate 

 balance between, all the cosmic forces; 

 and every living thing has just as many 

 and just as varied contacts as has a 

 butterfly. 



These simple facts, which seem almost 

 self evident, must constantly be kept in 

 mind in considering organic evolution. 



REPRODUCTION AS A PHASE OF GROWTH 



Another fact to be constantly remem- 

 bered is that life is the ability inherent in 

 so-called living things to increase in bulk 

 indefinitely. With the delicate balance 

 that exists between all living things and 

 their environment, their dependence on 

 certain restricted types of food to main- 

 tain the necessary chemical reactions and 

 on delicate physical adjustments for the 

 securing of that food, and to avoid serving 

 as food for something else, how can this 

 be brought about? 



In the organic world the individual 

 animal or plant is simply a phase in the 

 unbroken continuity in the increase of 

 living substance. The adult animal is 

 always so adjusted to the environment in 

 which it lives that it accumulates a 

 surplus of its special complex of organic 

 compounds. No adult animal can grow 

 beyond a certain size without increasing 

 maladjustment to the forces by which it is 

 strictly circumscribed. But it can accum- 

 ulate and store up material over and above 

 its own special needs. 



This material is released in the form of 

 eggs or young or buds, or in some other 

 form at or near the bottom of the scale 

 through which the individual passes dur- 

 ing its life cycle. Once it is released, this 

 surplus substance, in the form of numerous 

 units, runs the same course as the parent 

 organism, resulting in an increase in the 

 organic substance. 



This oscillation through a wide range of 

 limiting conditions, from eggs or young 

 to adults, by which increase in total mass 

 is effected by all living things, is extremely 

 complicated. Its simplest form is seen 

 in those minute creatures known as 

 protozoans, which, when the maximum 

 size possible for them is reached, simply 

 divide in two; each of the two derivatives 

 grows to the maximum efficient size, and 

 then again divides. Its most complex 

 form is to be observed in certain flies 

 (Hippoboscidas and Glossina) in which the 

 females produce maggots, which imme- 

 diately form a pupa, so that there is no 

 pre-adult feeding stage. 



In the first case, that is, among the pro- 

 tozoans, the growing young feed from the 

 very first on the same substances that 

 support the parents. In the second case, 

 among the viviparous flies, the young 

 are fed throughout their entire pre-adult 

 existence, on special secretions formed 

 within the body of the mother. 



Between these two extremes there is 

 every possible intermediate. The mam- 

 mals — dogs, cats, etc. — and many other 

 forms of life approach the hippoboscid 

 flies; they are fed by a secretion from the 

 mother until they are sufficiently devel- 

 oped to gather their own food, which in 

 these cases is always like that of the adult 

 stage. In other types of life, as illustrated 

 by the reptiles and the gallinaceous birds, 

 the egg is provided with a supply of food 

 material sufficient to carry the young up to 

 a stage when they can find or capture and 

 assimilate the normal adult food. In 

 still other forms of life, as especially in 

 certain birds, the young are capable of 

 feeding only on animal substances, while 

 the adults are especially seed eaters. On 

 the other hand, the tadpoles of our com- 

 mon frogs feed on vegetable detritus, but 

 the adults feed on living animals, par- 

 ticularly insects. Some creatures feed 

 only in the younger stages, and the adult 



