The concept of 'race' in humans is a complex and often contentious topic. From a biological perspective, the idea of distinct human races, as traditionally understood, is largely discredited. Genetic variation in humans is continuous and clinal, meaning traits change gradually across geographic space, rather than occurring in discrete, bounded categories. However, if we interpret 'formation of races' as the historical development of observable physical differences and regional genetic patterns, both heredity (genetics) and environment have played significant roles.

Heredity (Genetics): Genetic variation is the raw material for evolutionary change. Over long periods, different human populations, often geographically isolated, experienced varying selective pressures, leading to differences in allele frequencies. These genetic differences, passed down through heredity, contributed to the development of certain physical traits. For example:

1. Adaptation to UV Radiation: Skin color is a classic example. Populations living closer to the equator, exposed to high levels of UV radiation, developed darker skin due to higher melanin production, which protects against folate degradation and skin cancer. Conversely, populations in higher latitudes evolved lighter skin, facilitating Vitamin D synthesis in low UV environments.
2. Body Build and Climate: Genetic adaptations related to thermoregulation, such as body shape and limb proportions (e.g., Bergmann's and Allen's rules), are heritable traits that evolved in response to cold or hot climates.
3. Disease Resistance: Genetic traits conferring resistance to local diseases, such as the sickle cell trait in malaria-prone regions, became more prevalent through natural selection.
4. Genetic Drift and Gene Flow: Random changes in gene frequencies (genetic drift) in small, isolated populations, and the movement of genes between populations (gene flow), also contributed to the unique genetic profiles observed in different groups.

Environment: The environment acts as the selective agent, shaping which heritable traits are advantageous and thus become more common in a population. Environmental factors include:

1. Climate and Geography: Temperature, humidity, altitude, and the intensity of UV radiation directly influenced the evolution of traits like skin color, hair texture, and body morphology.
2. Diet and Subsistence: The availability of specific food sources and dietary practices led to adaptations such as lactose tolerance in populations with a history of dairy farming.
3. Pathogen Load: The presence of specific infectious diseases in an environment exerted strong selective pressure, leading to genetic adaptations for disease resistance.
4. Geographic Isolation: Natural barriers like mountains, oceans, and deserts limited gene flow between populations for extended periods, allowing local adaptations to become more pronounced and distinct.

Conclusion: While heredity provides the mechanism for passing on traits and environment provides the selective pressure, it is crucial to understand that these factors led to clinal variations, not discrete 'races.' Human populations have always been interconnected through migration and gene flow, preventing the formation of truly isolated, genetically distinct groups. The concept of race is primarily a social construct, often based on superficial phenotypic traits, rather than a biologically meaningful classification of human genetic diversity. Modern genetics emphasizes the vast genetic similarity among all humans and the continuous nature of human variation.