3 68 THE POPULAR SCIENCE MONTHLY. 



more or less exactness, to the ascending series of now living forms ; 

 showing, however, in addition to these, many connecting links between 

 existing classes, which, in the progress of time and development, have 

 diverged widely from each other ; while the modern science of em- 

 bryology as clearly shows that the development of the human being 

 beginning in a formless, structureless, microscopic speck of protoplasm, 

 comparable in all appreciable respects to the " dawn-animal " of the 

 Palaeozoic period, and to the moneron and amoeba of to-day consists 

 in the ascent, step by step, with a good degree of exactness, both of 

 the geological ladder and of the trunk of the animal tree whose branches 

 represent all existing forms of animal life, whose roots are deeply im- 

 bedded in the inorganic crust of the earth, and at whose apex appears 

 the genus homo the crown and consummate flower of organic devel- 

 opment. In other words, the individual development of every human 

 embryon is a brief resume (in which, it is true, some of the chapters 

 are suppressed and others greatly condensed) of the history of the de- 

 velopment of animal life on the globe, from its infancy to the present 

 day. 



In 1862 Professor Graham pointed out the importance of the two 

 states of matter, described by him as crystalloid and colloid^ crystal- 

 like and jelly-like. He says : "The colloidal state is, in fact, a dy- 

 namical state of matter ; the colloid possesses energy, and may be 

 regarded as the primary source of the forces appearing in the phenom- 

 ena of vitality." 



Although certain colloids have a very simple chemical composition 

 (as silica, for example, which, ordinarily crystalloid, is capable of ex- 

 isting in a colloidal state), the molecular constitution of the colloids in 

 general is undoubtedly highly complex. The molecule of albumen (a 

 typical colloid closely resembling protoplasm), while it consists of but 

 six different chemical elements, is estimated as containing several 

 hundred atoms of these elements, which thus render the molecule an 

 extremely massive one. 



Now this massiveness of its molecules confers upon protoplasm a 

 certain mechanical stability favorable to the preservation of organic 

 forms ; at the same time endowing it with the chemical instability 

 essential for the constant exchanges of material which constitute nu- 

 trition and are characteristic of all living matter. 



These massive molecules are also reservoirs of vast amounts of en- 

 ergy of the kind long known as potential a term which, though likely 

 to vanish, not into thin air but into the thinner ether, is nevertheless 

 a very convenient one. This potential energy, stored up during the 

 slow processes of plant-life and appropriated by animals in the form of 

 food, is liberated or manifested as actual energy in the decomposition 

 of their tissues that is, in the gradual breaking down of tissue-cells, 

 by means of which the animal functions are performed, and carbonic 

 acid, urea, and other excretory compounds are produced. 



