The main theme of the work is that, in the course of their embryonic development, all animals, including man, pass roughly and rapidly through a series of forms which represents the succession of their ancestors in the past. After a severe and extensive study of embryonic phenomena, Haeckel has drawn up a “law” (in the ordinary scientific sense) to this effect, and has called it “the biogenetic law,” or the chief law relating to the evolution (_genesis_) of life (_bios_). This law is widely and increasingly accepted by embryologists and zoologists. It is enough to quote a recent declaration of the great American zoologist, President D. Starr Jordan: “It is, of course, true that the life-history of the individual is an epitome of the life-history of the race”; while a distinguished German zoologist (Sarasin) has described it as being of the same use to the biologist as spectrum analysis is to the astronomer. But the reproduction of ancestral forms in the course of the embryonic development is by no means always clear, or even always present. Many of the embryonic phases do not recall ancestral stages at all. They may have done so originally, but we must remember that the embryonic life itself has been subject to adaptive changes for millions of years. All this is clearly explained by Professor Haeckel. For the moment, I would impress on the reader the vital importance of fixing the distinction from the start. He must thoroughly familiarise himself with the meaning of five terms. _Biogeny_ is the development of life in general (both in the individual and the species), or the sciences describing it. _Ontogeny_ is the development (embryonic and post-embryonic) of the individual (_on_), or the science describing it. _Phylogeny_ is the development of the race or stem (_phulon_), or the science describing it. Roughly, _ontogeny_ may be taken to mean embryology, and _phylogeny_ what we generally call evolution. Further, the embryonic phenomena sometimes reproduce ancestral forms, and they are then called _palingenetic_ (from _palin_ = again): sometimes they do not recall ancestral forms, but are later modifications due to adaptation, and they are then called _cenogenetic_ (from _kenos_ = new or foreign). These terms are now widely used, but the reader of Haeckel must understand them thoroughly.