The same may be said in regard to the use of metals. The invention of metallurgy, which marks so important an advance of European civilization, does not appear associated with analogous levels of development in other parts of the world. Similar remarks may be made in regard to the development of agriculture and of the domestication of animals. People whom in a general way we ought to class as on the same level of culture may some possess the art of agriculture, others may have domesticated animals, while still others may rely upon the bounty of the sea or upon the natural vegetable products of their home.[7] As soon as we begin to investigate the industrial achievements of different types belonging to different races, parallelism of industrial development does not seem to exist in any degree of detail. Only one general trait of industrial development remains; namely, the constant addition of new elements to the older stock of knowledge and an increasing refinement of methods and of results, setting aside periods of temporary regression. Thus it does not seem to be certain that every people in an advanced stage of civilization must have passed through all the stages of development, which we may gather by an investigation of all the types of culture which occur all over the world. A still more serious objection is based on another observation. The validity of the general sameness of the evolution of mankind is based on the assumption that the same cultural features must always have developed from the same causes, and that all variations are only minor details of the grand uniform type of evolution. In other words, its logical basis is the assumption that the same ethnical phenomena are always due to the same causes. Thus the inference in regard to the sequence of maternal and paternal institutions, to which I referred before, is based on the generalization that because in a few cases paternal families have developed from maternal ones, therefore all paternal families have developed in the same way. If we do not make the assumption that the same phenomena have everywhere developed in the same way, then we may just as well conclude that paternal families have in some cases arisen from maternal institutions, in other cases in other ways. In the same way it is inferred that because many conceptions of the future life have evidently developed from dreams and hallucinations, all notions of this character have had the same origin. This is true only if it can be shown that no other causes could possibly lead to the same ideas. To give another example. It has been claimed that among the Indians of Arizona, pottery developed from basketry, and it has been inferred that all pottery must therefore be later in the cultural development of mankind than basketry. Evidently this conclusion cannot be defended, for pottery may develop in other ways.