mrh@cybvax0.UUCP (Mike Huybensz) (02/25/85)
Ray Miller has written a scoffing criticism of a maligned and misunderstood phrase. I'm going to try to explain the current understanding of "Ontogeny recapitulates Phylogeny." First, let's get straight that "ontogeny recapitulates phylogeny" is a bumper-sticker bastardization of what Haeckle really said. It is too short and pithy to be exact. Another example of such a summarization concerns the philosophical debate over which is primary, mind or matter (from roughly the same period.) "What is mind? No matter." "What is matter? Never mind." Next, let's translate it into non-technical English. As a first, rough translation, let's try "During development into an adult, an organism passes through stages resembling the organisms from the root of the evolutionary tree up to the presently occupied branch tip." This first translation is obviously too vague. During those stages, would the organism resemble the adults? Obviously not-- at no point in their development do humans look like little adult fishes or reptiles. A more appropriate translation is "during an individual's devlopment, it will pass through stages that its ancestors passed through." So, in our embryonic and fetal development, we will pass through stages resembling embryonic and fetal stages of fishes and reptiles (and other ancestral taxa.) This concept powered major studies of the development of a wide variety of organisms in a quest to determine which evolved from what. A vast new range of embryologic information was discovered, which is still much used today. Not just gill slits were discovered: the progressive development of the chambers of the heart, basic patterns of cleavage of eggs, the fundamental tube-within-a-tube structure we have in common with many invertebrates, etc. The one obvious question is why this should be so. Why would developmental stages of ancestors be retained? The answer is that sometimes they are, and sometimes they are not. One important reason why they might be retained is because development is a step-by-step building process. Loss of one of the steps might be fatal. Adding new steps might not. One reason why they may not be retained is if a step is modified. Eventually the sum of modifications may be sufficient to conceal the original path of development. There are more possibilities still. Development of an organism consists of many steps occurring in series and parallel. Changing the order of the steps may also make the process different from that of an ancestor. I don't know how well Haeckle understood these points. Nowadays, the term "ontogeny recapitulates phylogeny" is rather useless, as it is too terse to be exact, and too catchy to be forgotten. -- Mike Huybensz ...decvax!genrad!mit-eddie!cybvax0!mrh