At the very threshold of the inquiry stands a grand difficulty. Strange as it may seem, it is nevertheless true, that notwithstanding the many years, the immense labour bestowed by illustrious men on this subject, next to nothing is known of the relations existing between the lowest forms of life, animal or vegetable (especially the latter), as well as their germs, and varying physical and chemical conditions. Heat, light, cold and darkness, alkalies and acids with other chemical compounds, one would imagine to be not without their influence. Yet what this influence may be in a given case, none can tell. Enough is known in the way of a few detached facts to make it certain that these agents have very decided effects. It might be thought that any one who wished to attack the problem of Spontaneous Generation anew would first try to obtain some connected knowledge on this point. Indeed, until it has been cleared up somewhat, it is not very evident what good Experimentation on Heterogenesis can do. It is much as if a chemist were to throw a handful of stuff (what he knew not) into a crucible, and then expect a valuable result. It can scarcely be doubted that many of these lower organisms live and grow under conditions which _à priori_ might seem incompatible with vitality. It is clear also that the work of one experimenter ought to be such as may admit of repetition by another with the same result. Now no one who attempts the study of this subject of Spontaneous Generation, can fail to be struck with the immense mass of conflicting evidence. Some mischievous spirit appears to have purposely thrown confusion over the whole. Facts are alternately stated and denied. It is very hard to be sure of the right road, even for an experienced person. Another point relates to the value of the evidence when obtained. It can scarcely be doubted that out of a given number of experiments undertaken to establish a case of Heterogenesis or Archebiosis, great value should be attached to the negative evidence afforded by those that disprove the supposed fact. A little consideration will show the reason. The precautions to be taken against the intrusion of germs are innumerable: a slight exposure to the air, accidental contact of an unheated rod or tube, or neglect of some other particulars may inoculate the experimental fluid. Hence even with care the chances are many in favour of some such accidental intrusion, and great caution should always be exercised before an affirmative result can be regarded as firmly established.