Organic evolution refers to the gradual changes that occurs in living organisms over time. It is the result of change in the genetic makeup of the organism due to mutation and other sources of variations. The favourable mutations are accepted by nature and provides the organism an adaptive advantage, leading to its evolution.
Evolution cannot always be equated to progress because it does not always leads to the formation of a new species. In fact, most of the times, it leads to the generation of diversity. One species does not necessarily gets eliminated to give use to a new ones in prevailing environments. Also, the newly formed species may have complex organisation because of evolution but it cannot be considered better than the earlier species. Many older and simpler designs still survive in the nature. For example, bacteria are one of the simplest life forms on earth and still they can survive in the most adverse conditions, such as hot springs, deep sea thermal vents etc. Therefore, having more complex body designs does not make any species superior to others.