In this table the individual families are not mentioned, because for the present we need only the main outline of classification to illustrate the principal facts about the course of evolution. As the table is given, the simplest families come first, the succeeding ones gradually increasing in complexity till the last group represents the most advanced type with which we are acquainted, and the one which is the dominant group of the present day. This must not be taken as a suggestion that the members of this series have evolved directly one from the other in the order in which they stand in the table. That is indeed far from the case, and the relations between the groups are highly complex. It must be remarked here that it is often difficult, even impossible, to decide which are the most highly evolved members of any group of plants. Each individual of the higher families is a very complicated organism consisting of many parts, each of which has evolved more or less independently of the others in response to some special quality of the surroundings. For instance, one plant may require, and therefore evolve, a very complex and well-developed water-carriage system while retaining a simple type of flower; another may grow where the water problem does not trouble it, but where it needs to develop special methods for getting its ovules pollinated; and so on, in infinite variety. As a result of this, in almost all plants we have some organs highly evolved and specialized, and others still in a primitive or relatively primitive condition. It is only possible to determine the relative positions of plants on the scale of development by making an average conclusion from the study of the details of all their parts. This, however, is beset with difficulties, and in most cases the scientist, weighed by personal inclinations, arbitrarily decides on one or other character to which he pays much attention as a criterion, while another scientist tends to lay stress on different characters which may point in another direction. In no group is this better illustrated than among the Coniferæ, where the relative arrangement of the different families included in it is still very uncertain, and where the observations of different workers, each dealing mainly with different characters in the plants, tend to contradict each other. This, however, as a byword. Notwithstanding these difficulties, which it would be unfair to ignore, the main scheme of evolution stands out clearly before the scientist of to-day, and his views are largely supported by many important facts from both fossil and living plants.