AN OUTLINE OF THE THEORY. 23 
mentary, being but a remnant of the much longer and 
more complex appendix of the same nature in living 
animals today. 
2. Embryonic Development. Because the young of 
all animals resemble one another while in the embryo 
stage, and since such resemblances are found in man, it 
is concluded that the evolution of man from some related 
animal form must be accepted as the most: reasonable 
explanation. 
3. Some diseases are common to animals and man 
(tuberculosis, cholera, hydrophobia, etc.). 
4. The similarity in structure of man and the apes. 
5. The fossil remains of man. Certain skulls and 
leg bones have been found which are said to represent 
forms higher than the ape and lower than man. On the 
strength of such finds it is said that the “missing link” 
has now been supplied. 
The Nebular Hypothesis. 
The Frenchman de La Place (1827) first promul- 
gated in modern terminology the theory once held by 
Greek philosophers, that the*earth and the system in 
which it is a member originated from a primitive cosmic 
vapor or universal fire-mist filling all space with infinitely 
small atoms. In this homogeneous mass motion origi- 
nated, resulting in a concentration at one point. This 
condensation resulted in heat and light. The planetary 
system at first consisted of a huge gas-ball which gradu- 
ally cooled, contracting into a molten mass which under 
the influence of centrifugal force began to rotate. This 
rotation became more rapid as the mass condensed, throw- 
ing off the planets, in which the process was repeated 
(the moons being cast off), until the earth became suffi- 
ciently cool to sustain life. 
