14 EVOLUTION. 



Aristotle first maintained that the chick developed 

 from a little speck on the surface of the yolk of the 

 egg ; and Harvey, discoverer of the circulation of the 

 blood, declared that the forms were gradually devel- 

 oped from this speck through the action of the blood. 



The great discovery of more recent times in this 

 department is the fact, that the germs of all animals 

 appear to be alike in their earliest stage, and they 

 develop through similar forms until they assume the 

 special features of their own species. The embryo 

 of each animal successively resembles the embryos 

 of the races below it, until they part company to attain 

 their final form. 



The lowest forms of life, the protozoans, repro- 

 duce themselves by division of their oodies, or the 

 formation of buds, but all other animals produce eggs, 

 which, when fertilized, divide and sub-divide till they 

 form a mass of cells like a mulberry. The germ then 

 gradually changes info a cup-form, called gastrtda, 

 having two different layers, called the primary-germ 

 layers. The germs of all invertebrates pass through 

 these changes, as do also those of the vertebrates, as 

 far as observed. Other changes occur which are 

 common to all animal germs, each one passing through 

 the forms common to the races below it, and then 

 diverging to its own peculiarities. 



The development of man, as far as traced, agrees 

 with that of lower animals : the morula or mulberry 

 stage is shown, as in the sponge, sea-squirt, and 



