Body Covering 45 



at some period of their infancy, a series of not very clearly 

 defined, oval, bluish-gray patches along their sides. In the 

 California golden trout and a similar Canadian form, both 

 inhabitants of small streams in high mountain regions, these 

 parr-marks often remain visible throughout life, but in other 

 species they vanish before maturity is reached. The theory is 

 that these markings were permanent features of the extinct 

 ancestors of the salmon family, and that in the course of 

 evolution they were pushed out of the picture until now the 

 only chance they get to appear is during the early life of 

 each individual. This is called the recapitulation theory, or 

 the biogenetic larxj and we shall meet it again in the course 

 of this book. It is a theory devised by biologists for their own 

 edification and enlightenment. It holds that the develop- 

 mental history of the individual recapitulates the evolution- 

 ar}' history of the species. In more but shorter words, each 

 individual is supposed to go through, during its growth and 

 more especially during its early embryonic period, stages 

 which resemble various features characteristic of the species 

 which were its evolutionary^ ancestors. For instance, the gill 

 slits which are present for a short while in the early human 

 embr\^o are supposed to represent the fish-stage of our evolu- 

 tionary history. It is an h^-pothesis rather than a law, and in 

 spite of much evidence in its favor it is incapable of con- 

 clusive proof, but biologists delight in hauling it out, when- 

 ever opportunity^ offers, to see how it matches up with the 

 facts. As for the developmental color-changes which led us 

 off on this side-track, it may be assumed that they are due to 

 slow alterations in the permanent pigment pattern, and not 

 to chromatophores. 



What happened to chromatophores in the course of evolu- 

 tion? Why can't we change our color? To be sure, we get 

 red when angr\^, pale when frightened, but that is due only 

 to the change in the amount of blood in our surface capil- 



