An abler work than any of these, but exhibiting less power of observation is a treatise, περι γονης {peri gonês}, _On generation_, that may perhaps be dated about 380 B. C.[13] It exhibits a writer of much philosophic power, very anxious for physiological explanations, but hampered by ignorance of physics. He has, in fact, the weaknesses and in a minor degree the strength of his successor Aristotle, of whose great work on generation he gives us a fore-taste. He sets forth in considerable detail a doctrine of pangenesis, not wholly unlike that of Darwin. In order to explain the phenomena of inheritance he supposes that vessels reach the seed, carrying with them samples from all parts of the body. He believes that channels pass from all the organs to the brain and then to the spinal marrow (or to the marrow direct), thence to the kidneys and on to the genital organs; he believes, too, that he knows the actual location of one such channel, for he observes, wrongly, that incision behind the ears, by interrupting the passage, leads to impotence. As an outcome of this theory he is prepared to accept inheritance of acquired characters. The embryo develops and breathes by material transmitted from the mother through the umbilical cord. We encounter here also a very detailed description of a specimen of exfoliated _membrana mucosa uteri_ which our author mistakes for an embryo, but his remarks at least exhibit the most eager curiosity.[14] [13] The three works περι γονης {peri gonês}, περι φυσιος παιδιου {peri physios paidiou}, περι νουσων δ {peri nousôn d}, _On generation_, _on the nature of the embryo_, _on diseases, book IV_, form really one treatise on generation. [14] περι φυσιος παιδιου {peri physios paidiou}, _On the nature of the embryo_, § 13. The same experience is described in the περι σαρκων {peri sarkôn}, _On the muscles_. The author of this work on generation is thus a 'biologist' in the modern sense, and among the passages exhibiting him in this light is his comparison of the human embryo with the chick. 'The embryo is in a membrane in the centre of which is the navel through which it draws and gives its breath, and the membranes arise from the umbilical cord.... The structure of the child you will find from first to last as I have already described.... If you wish, try this experiment: take twenty or more eggs and let them be incubated by two or more hens. Then each day from the second to that of hatching remove an egg, break it, and examine it. You will find exactly as I say, for the nature of the bird can be likened to that of man. The membranes [you will see] proceed from the umbilical cord, and all that I have said on the subject of the infant you will find in a bird's egg, and one who has made these observations will be surprised to find an umbilical cord in a bird's egg.'[15] [15] περι φυσιος παιδιου {peri physios paidiou}, _On the nature of the embryo_, § 29.