or putrescent matter determines at once the germination of the always-present spore.  But a new question arises.  These spores are definite products.  In the face of some experimental facts one was tempted to inquire:  Have these spores any capacity to resist heat greater than the adults?  It was not easy to determine this question.  But we at length were enabled to isolate the germs of seven separate forms, and by means of delicate apparatus, and some twelve months of research, to place each spore sac in an apparatus so constructed that it could be raised to successive temperatures, and without any change of conditions examined on the stage of the microscope.

In this way we reached successive temperatures higher and higher until the death point—­the point beyond which no subsequent germination ever occurred—­was reached in regard to each organism.  The result was striking.  The normal death point for the adult was 140 deg.  F. One of the monads emitted from its sac minute mobile specks—­evidently living bodies—­which rapidly grew.  These we always destroyed at a temperature of 180 deg.  F. Three of the sacs emitted spores that germinated at every temperature under 250 deg.  F. Two more only had their power of germination destroyed at 260 deg.  F. And one, the least of all the monad forms, in a heat partially fluid and partially dry, at all points up to 300 deg.  F. But if wholly in fluid it was destroyed at the point of 290 deg.  F. The average being that the power of heat resistance in the spore was to that of the adult as 11 to 6.  From this it is clear that we dare not infer spontaneous generation after heat until we know the life-history of the organism.

In proof of this I close with a practical case.  A trenchant and resolute advocate of the origin of living forms de novo has published what he considers a crucial illustration in support of his case.  He took a strong infusion of common cress, placed it in a flask, boiled it, and, while boiling, hermetically sealed it.  He then heated it up in a digester to 270 deg.  F. It was kept for nine weeks and then opened, and, in his own language, on microscopical examination of the earliest drop “there appeared more than a dozen very active monads.”  He has fortunately measured and roughly drawn these.  A facsimile of his drawing is here.  He says that they were possessed of a rapidly moving lash, and that there were other forms without tails, which he assumed were developmental stages of the form.  This is nothing less than the monad whose life-history I gave you last.  My drawings, magnified 2,500 diams., of the active organism and the developing sac are here.