to know that in growing up a human infant undergoes the changes of childhood and adolescence, that kittens become cats, and that an oak tree is produced by an acorn, for we know these things directly by observing them.  It is natural for development to take place under normal conditions, and if it does not, then something has interfered with nature.  Inasmuch as “growing up” is accomplished by the alteration of an organic mechanism with one structure into an individual with a changed plan of body, it is in essence the actual process of evolution which the comparative study of grown animals of to-day demonstrates in the way we have learned.  The study of animal structure discovers the process of evolution because the most reasonable interpretation of the similarities and minor differences exhibited everywhere by the various groups of animals is that descent with adaptive and divergent modification has taken place; the result is reached by inference, it is true, but by scientific and logical inference.  With development it is otherwise.  No reasoning is necessary to tell us that organic transformation is a real and a natural process.  We see it everywhere about us and we ourselves have come to be what we are by a natural history of change.  Can we consistently deny that it is possible for a species to alter in the long course of time when a few brief weeks are sufficient for the new-laid egg of the fowl to develop into a fledgling?  Many indeed strain at the gnat of the longer process in the past when without hesitation they recognize the real and obvious fact of individual development in a brief period.

I have said that development is a “natural” process.  We employ this word for the familiar and everyday occurrence or thing; it does not imply that everything is known about the object or phenomenon, because science knows that complete and final knowledge is impossible.  We say that it is natural for rain to fall to the earth, and we speak of the law of gravitation according to which this takes place as a natural principle, but it may not have occurred to many to inquire what makes rain fall and why do masses of matter everywhere behave toward one another in the consistent manner described by the law in question.  Sunshine is natural, but we do not know why light travels as it does from the sun to the earth, and this is another question which, like the inquiry into the ultimate cause of the familiar and natural phenomenon of gravitation, has not yet been answered.  But it is still regarded as natural for the rain to fall and for the sun to shine.  In the same way does science view development, denoting it natural because it is an ordinary everyday matter.  And we are under no more obligation to postulate supernatural control for the changing forms in the life-history of a chick or a cat than we need to assume that gravitation and the radiation of light demand immediate supernatural direction.  The embryology of no form is fully understood or described