It is a common mistake with those who pursue philosophical studies in these times, to seek for the cause of diversity of parts in diversity of the matter whence they arise. Thus medical men assert that the several parts of the body are both engendered and nourished by diverse matters, either the blood or the seminal fluid; viz. the softer parts, such as the flesh, by the thinner matter, the harder and more earthy parts, such as the bones, &c. by the firmer and thicker matter. But we have elsewhere refuted this too prevalent error. Nor do they err less who, with Democritus, compose all things of atoms; or with Empedocles, of elements. As if _generation_ were nothing more than a separation, or aggregation, or disposition of things. It is not indeed to be denied, that when one thing is to be produced from another, all these are necessary, but generation itself is different from them all. I find Aristotle to be of this opinion; and it is my intention, by-and-by, to teach that out of the same albumen (which all allow to be uniform, not composed of diverse parts,) all the parts of the chick, bones, nails, feathers, flesh, &c. are produced and nourished. Moreover, they who philosophize in this way, assign a material cause [for generation], and deduce the causes of natural things either from the elements concurring spontaneously or accidentally, or from atoms variously arranged; they do not attain to that which is first in the operations of nature and in the generation and nutrition of animals; viz. they do not recognize that efficient cause and divinity of nature which works at all times with consummate art, and providence, and wisdom, and ever for a certain purpose, and to some good end; they derogate from the honour of the Divine Architect, who has not contrived the shell for the defence of the egg with less of skill and of foresight than he has composed all the other parts of the egg of the same matter, and produced it under the influence of the same formative faculty. Although what has already been said be the fact, namely, that the egg, even whilst contained in the uterus, is provided with a hard shell, still the authority of Aristotle has always such weight with me that I never think of differing from him inconsiderately; and I therefore believe, and my observations bear me out in so much, that the shell does gain somewhat in solidity from the ambient air upon its extrusion; that the sluggish and slippery fluid with which it is moistened when laid, immediately becomes hardened on its exposure to the air. For the shell, whilst the egg is in the uterus, is much thinner and more transparent, and smoother on the surface; when laid, however, the shell is thicker, less translucid, and the surface is rough--it appears as if it were powdered over with a fine white dust which had but just adhered to it. Let us, as we are upon this subject, expatiate a little:--