72 FOOT-NOTES TO EVOLUTION. 
proof, insusceptible of refutation. The argument for it 
is chiefly this: Life exists on a globe once lifeless. How 
did life begin? If not through spontaneous generation, 
how did it come? Must it not have been by the opera- 
tion of those laws and forces which through all time 
change lifeless into living matter? Very likely, but we 
do not know. We know nothing whatever of such laws 
and forces, and we gain nothing by veiling our ignorance 
under a philosophical necessity. 
Moreover, if spontaneous generation occurs as a re- 
sultant of any forces, like forces would produce it again. 
We have never known it to occur. Should it occur, the 
organisms thus produced would have no bonds of blood 
relationship with those already in existence. With these 
they should show no homology, as they could have no 
inheritance in common. But all known organisms have 
common homologies. The factors of organic evolu- 
tion are essentially the same for all. The unity of life 
amid all its diversity seems to point to origin froma 
common stock. If not from one stock, the lines of 
division between one and another are hidden from us, 
The study of embryology breaks down the time-honoured 
branch lines of vertebrates, articulates, molluscs, and 
radiates. The groups of animals are more numerous, 
more complex, and more intertangled than Cuvier and 
Agassiz thought. The number of primary branches of 
animals or plants is uncertain, their boundaries unde- 
fined. 
If spontaneous generation exists, it is a factor in 
evolution. If it is a factor, our explanation of the 
meaning and nature of homology must be fundamentally 
changed. But it may be that it should be changed. We 
can not show that spontaneous generation does not 
exist. All we know is that we have no means of recog- 
nising it. If there is now spontaneous generation of 
