Within the context of evolution, “orthogenesis” is defined as “the doctrine that evolution is governed by intrinsic factors and occurs in predictable directions.” [www.biology-online.org/dictionary/Orthogenesis] A simpler and more useful definition would be that orthogenesis refers to evolution as occurring in a linear, incremental fashion involving transitions that follow logically from one another. Developed as a theory by late-19th Century German zoologist Wilhelm Haacke, “orthogenesis” was defined by Haacke’s fellow zoologist Theodor Eimer as “the general law according to which evolutionary development takes place in a noticeable direction . . .”
“Orthogenesis,” as can be easily seen, is comprised of two words, “ortho” and “genesis,” the former referring to “straightness” and the latter, of course, to origins or beginnings. Orthogenesis, then, refers to the unilinear path down which a species has evolved into its current manifestation. The theory or application of orthogenesis to human and other species has been disputed by many contemporary anthropologists and others who have determined that evolution has not occurred in a straight line manner but rather in a more disparate way involving different branches or paths down which a certain species has evolved into more than one variation of its original self. Scientists at the University of Texas, El Paso, for instance, have cited the evidence of the evolution of horses to dispute the theory of orthogenesis with respect to evolutionary processes:
“In the late 1800s and early 1900s, some biologists thought that evolution occurred in a straight line without deviation, as if evolving toward a goal. Thus the idea that early horses were destined to lose toes and develop increasingly complex grinding teeth until Lo!, they became today’s horses. Fortunately, orthogenesis was a testable hypothesis – and when tested, failed. We now realize that horse evolution was more like a brush than an arrow. Only one twig made it to the present, but evidence of a wide variety of horses is plain in the fossil record. Look closely at our Chihuahuan Desert fossil beds, and old three-toes is with us nearly to the end.” [www.museum2.utep.edu/archive/biology/DDorthogenesis.htm]
“Orthogenesis,” then, is a largely-discredited theory of evolution as occurring in a unilinear, predictive manner involving one, and only one, variation of a species as having evolved from the previous manifestations of that species.