144 



THE WONDERFUL CENTURY. 



The second great generalization is what has been 

 termed the recapitulation theory of development. 

 Every animal or plant begins its existence as a cell, 

 which develops by a process of repeated fission and 

 growth into the perfect form. But if we trace the dif- 

 ferent types backward, we find that we come to a stage 

 when the embryos of all the members of an order, such as 

 the various species of Ruminants, are undistinguishable ; 

 earlier still all the members of a class, such as the Mam- 

 malia, are equally alike, so that the embryos of a sheep 

 and a tiger would be almost identical; earlier still all ver- 

 tebrates, a lizard, a bird, and a monkey, are equally un- 

 distinguishable. Thus in its progress from the cell to 

 the perfect form every animal recapitulates, as it were, 

 the lower forms upon its line of descent, thus affording 

 one of the strongest indirect proofs of the theory of evo- 

 lution. The earliest definite result of cell-division is to 

 form what is termed the " gastrula," which is a sack 

 with a narrow mouth, formed of two layers of cells. 

 All the higher animals without exception, from mollusk 

 to man, go through this " gastrula " stage, which again 

 indicates that all are descended from a common ancestral 

 form of this general type. 



One other physiological discovery is worth noting 

 here, both on account of its remarkable nature and be- 

 cause it leads to some important conclusions in relation 

 to the zymotic diseases. Quite recently it has been 

 proved that the white corpuscles of the blood, whose 

 function was previously unknown, are really independ- 

 ent living organisms. They are produced in large num- 

 bers by the spleen, an organ which has long been a 

 puzzle to physiologists, but whose function and impor- 



