94 AN INTRODUCTION TO SCIENCE 



consists of a tabulated mass of lymphoid tissue tissue, 

 that is to say, in which young white blood-corpuscles, or 

 leucocytes, are being formed. Little spherical nests of 

 epithelial cells are embedded in this tissue, as well as a 

 number of amorphous globules termed "fuchsin-bodies," 

 because they stain darkly with this dye. Now fuchsin- 

 bodies are found in the olfactory apparatus of the brain, 

 after it has begun to atrophy, and there can be no doubt 

 that they indicate cells, or blood, which have undergone 

 chemical change. The nests of epithelial cells are probably 

 the remains of gland tissue blocked-up ducts, we may 

 say, almost with certainty after studying the development of 

 the body. Here, then, is an organ which grows like a 

 gland (or like an organ of respiration), although it is not 

 found as a functional gland in any vertebrate. \Yhat is it 

 doing in Man ? It is a manuscript which cannot be read. 

 The characters in which it is written were obsolete before 

 the earliest fish came into existence. Why, then, has it been 

 retained? For the sake of the palimpsest, which we can 

 read. Like all other glands formed in connection with the 

 front part of the alimentary canal, it is surrounded by 

 lymphoid tissue. For the sake of this crossed writing it is 

 retained in every individual until he reaches an age at which 

 his great need of a nursery for young leucocytes has les- 

 sened. After that it disappears. 



It is not only in living things as they appear in the adult 

 condition that the biologist traces adaptation, now that the 

 law of evolution has been formulated, but in every stage of 

 growth. As he watches the changes through which the 

 single-celled ovum evolves into the fully grown animal, he 

 sees the race of which this particular species is the heir 

 passing through all the stages which have marked its his- 

 tory from age to age. In a few days, or weeks, or months, 

 a drama is acted which it has taken geological seons to 

 rehearse, for every individual recapitulates in its growth the 

 successive stages to which its ancestors attained, and at 



