I am the more encouraged to speak of the difficulty to which I refer, because I have reason to know that it presents itself again and again in the provincial societies of the country, and is often adhered to with a tenacity worthy of a better cause.  I refer to the danger that always exists, that young or occasional observers are exposed to, amid the complexities of minute animal and vegetable life, of concluding that they have come upon absolute evidences of the transformation of one minute form into another; that in fact they have demonstrated cases of heterogenesis.

This difficulty is not diminished by the fact that on the shelves of most microscopical societies there is to be found some sort of literature written in support of this strange doctrine.

You will pardon me for allusion again to the field of inquiry in which I have spent so many happy hours.  It is, as you know, a region of life in which we touch, as it were, the very margin of living things.  If nature were capricious anywhere, we might expect to find her so here.  If her methods were in a slovenly or only half determined condition, we might expect to find it here.  But it is not so.  Know accurately what you are doing, use the precautions absolutely essential, and through years of the closest observation it will be seen that the vegetative and vital processes generally, of the very simplest and lowliest life forms, are as much directed and controlled by immutable laws as the most complex and elevated.

The life cycles, accurately known, of monads repeat themselves as accurately as those of rotifers or planarians.

And of course, on the very surface of the matter, the question presents itself to the biologist why it should not be so.  The irrefragable philosophy of modern biology is that the most complex forms of living creatures have derived their splendid complexity and adaptations from the slow and majestically progressive variation and survival from the simpler and the simplest forms.  If, then, the simplest forms of the present and the past were not governed by accurate and unchanging laws of life, how did the rigid certainties that manifestly and admittedly govern the more complex and the most complex come into play?

If our modern philosophy of biology be, as we know it is, true, then it must be very strong evidence indeed that would lead us to conclude that the laws seen to be universal break down and cease accurately to operate where the objects become microscopic, and our knowledge of them is by no means full, exhaustive, and clear.

Moreover, looked at in the abstract, it is a little difficult to conceive why there should be more uncertainty about the life processes of a group of lowly living things than there should be about the behavior, in reaction, of a given group of molecules.

The triumph of modern knowledge is the certainty, which nothing can shake, that nature’s laws are immutable.  The stability of her processes, the precision of her action, and the universality of her laws, is the basis of all science, to which biology forms no exception.  Once establish, by clear and unmistakable demonstration, the life history of an organism, and truly some change must have come over nature as a whole, if that life history be not the same to-morrow as to-day; and the same to one observer, in the same conditions, as to another.