Through the course of evolution, the basic molecules of life (nucleic acids, proteins, carbohydrates, fats) have remained the same or have been conserved. Many organisms also share a lot of metabolic pathways. However the morphology (the form and structure of organisms) have changed considerably. Eg. - The appearance of hydra, fish, plants, sponges, humans etc. are considerably different.
This shows a greater stability of molecular structure when compared with morphological structures.