358 ANIMAL BIOLOGY. [Part II. 



dence would seem to be very largely in favour of biogenesis. 

 For if a solution be sterilised by boiling, sufficiently prolonged 

 to kill all germs, and then be preserved from contact with the 

 dust- and germ-laden air by a plug of cotton wool, the molecules 

 of the organic but lifeless matter in solution are utterly in- 

 capable of regrouping themselves into living forms, and the infu- 

 sion remains barren of life. This is the negative evidence in 

 favour of biogenesis. 



Day by day positive evidence in favour of the same view in- 

 creases in amount as the life-histories of these lowly forms of 

 life are patiently worked out, and it is found that they " breed 

 true." In Fig. 103, D., is figured a form of monad (Heteromita) 

 found in an infusion of cod's head. It has two whip-like pro- 

 cesses, a flagellum (/.) and a gubernaculum (gu.\ by which it 

 can anchor itself. There is an endoplast (n.\ and a contractile 

 vesicle (c. v.). Such monads multiply by fission, but also by an- 

 other process. Two individuals conjugate and fuse. A trian- 

 gular cyst (E.) results. After a while the cyst bursts, and a 

 homogeneous fluid is emitted (F.). The highest powers of the 

 microscope fail to disclose in this emitted matter any germ of 

 life, and there, at first sight, would seem to be an end of the 

 matter. But wait and watch, and there appear in the field of 

 the microscope, suddenly and as if by magic, countless minute 

 points which prolonged watching shows to be growing. And 

 when they have fully grown every distinct point is seen to be 

 a monad. Thus we see that a solution in which the highest 

 powers of the microscope disclose no trace of living matter, may 

 nevertheless be swarming with germs. Let a drop of such solu- 

 tion evaporate into the air ; there is no reason to suppose that 

 the germs perish. On the contrary, there is much to lead us to 

 believe that the germs have far greater powers of resisting high 

 temperature, desiccation, and other adverse conditions, than the 

 fully developed organism. We may thus see how the air comes 

 to be laden with germs which, should they fall into an appro- 

 priate infusion, may give rise to the teeming life which we know 

 to be so soon developed in it. 



Biogenesis would seem, then, to be a law of life under the 



