The rates of evolution can be compared on the basis of the number of quantitative changes that occur in a certain phylogenetic line within a given time interval. As various types of organisms differ in the number of traits that can be identified, it makes sense to use this means of estimating rates of evolution only to compare rates for similar organisms. This method is most useful for monitoring the changes in the rates of evolution within a single phylogenetic line. When, for example, this method was employed to follow the rate of evolution in lungfish, i.e. a group of vertebrates whose members appeared in the paleontological record approximately 340 million years ago, it was found that this rate achieved a maximum of 2.5 changes per million years at a time 290 million years ago, but decreased to about one tenth of this value 250 million years ago (Westoll 1949). Over the past 200 million years, the morphology of lungfish has practically not changed, so that their modern representatives are mostly given as examples of “living fossils”. Understandably, the rate of evolution in an evolutionary line depends not only on the rate of changes occurring in a single species, but also on the number of species in the given line. As, at the present time, there are only six known species of lungfish, it is not very surprising that the rate of evolution measured in terms of the number of evolutionary changes is very low at the present time.