Descriptive Biology 105 



Man begins his life as a single cell, corresponding to the 

 protozoan stage of evolution. Later lie consists of two layers 

 of cells, corresponding to a greatly moditied Hydra. Then 

 he possesses three "germ layers" and numerous segments like 

 a worm, from which stage he passes to that in which he has 

 gills like a fish, and later on a tail and coat of hair like a 

 monkey.^ These and many other facts of like nature have 

 led to the law that the development of the individual is a 

 replica in miniature of the development of the race, a law 

 which has been much abused by its friends no less than by 

 its enemies. While this principle is unquestionably sound it 

 must not be pushed too far. In its main features the develop- 

 ment of the individnal does reproduce that of the race, but 

 in all its details certainly not. At no time in his development 

 does man ever resemble an adult fish, but there are certain 

 stages in development which are common to both ; man in his 

 earliest stages having certain fish-like features. 



To follow the trail of animal evolution through the develop- 

 ment of the individual calls for all the cunning of a biological 

 Sherlock Holmes. Nature has, as it were, cleverly concealed 

 her footsteps, and made many a false move to throw the 

 pursuer off the track. But her trail, is there and can be 

 followed if we have the necessary acumen and patience. 



In the development of many worms, molluscs, echinoderms, 

 tunicates and other invertebrates there occur certain larval 

 forms known as trochophores. All trochophore larv^ are by 

 no means alike, but all have the same general plan of struc- 

 ture, and the presence of these larva; in the development of 

 so many different groups of invertebrates has led to a belief 

 in an ancestral "trochophore" from which these groups have 

 radiated, like the spokes about the hub of a wheel. 



Very closel.y resembling in many respects the trochophore 

 larva is the group of rotifers or "wheel animalcules" so called 

 because of the circles of rapidly beating cilia leading to 

 the mouth, which in their activity resemble the motion of 

 a wheel, and by means of which the rotifer obtains its food 

 and swims through the water. One of these, Trochospha^ra, 

 resembles the trochophore larva in its general form so closely, 

 as to have misled some biologists into the belief that this was 

 an ancestral type, and that the rotifers were the hub of the 

 animal universe. But the absence of a body cavity renders 

 such an interpretation more than doubtful, the rotifers hav- 

 ing more likely been shoved off onto a side track from the 

 main branches of evolution, where they stand, nearly related 



* This does not mean of course fully developed gills, tail and hair, 

 but their beginnings may be seen. If the development of a lish 

 or a mammal were arrested at a certain stage these organs would 

 not be better developed in them than in man. 



