Biologists of to-day approach the problem of organic development equipped with incomparably greater knowledge and with more delicate methods of research. But in our thoughts to-day, as we discuss the essential nature of the process of organic development and the mutual causal relations between rudiments and their products, the same contradictory views are present, altered only as our methods of expression have altered. In a striking fashion Roux[1] has contrasted the opposing ideas inherent in our modern conception of development, but yet identical with those which formerly found expression in the theories of preformation and epigenesis. By the term "embryonic development," in its ordinary acceptation, we understand the appearance of visible complexity. But when we speak of the visibility of the resulting complexity, we use a subjective term, the value of which is relative to the human eye. Going further into the matter, we must break up the conception into two parts, and distinguish between the actual production of complexity and the mere transformation of complexity from a condition invisible to us into complexity visible to our senses. 'The two kinds of development I have indicated bear a relation to each other that recalls the old opposing doctrines of preformation and epigenesis, the alternatives of a time when it was a task--perhaps the only possible task--to record the completed results of the stages in development as they became complete--in fact, to record the externally visible changes of shape. In this descriptive investigation of the development of external form, epigenesis, the successive formation of new shapes, gained a complete victory over evolution, the mere becoming visible of pre-existing details of shape. 'The closer investigation of embryonic development that is necessary in a search for causes brings us once more against the old alternatives, and compels us to a closer scrutiny of them. 'In this, if we still retain the old terms, epigenesis would mean not merely the building up of complicated form through the agency of a substratum, apparently simple, but perhaps with an extraordinarily complicated, minute structure, but, in the strictest sense of the term, the new formation of complexity, an actual increase of complexity. Evolution, on the other hand, would imply the mere becoming visible of pre-existing latent differentiation. Clearly, according to these general definitions, occurrences which outwardly exhibit epigenesis may be in reality partial or complete evolution. In fact, the deepest consideration leads us again to the original question: Is embryonic development epigenesis or evolution? Is it the new formation of complexity, or is it the becoming visible of complexity previously invisible to us?' Thus, in our own days, after the controversy has been at rest for long, biologists are assembled in opposing groups, one under the standard of epigenesis, another under that of preformation.